Depends largely on how it happens. There are a couple of PODs which would be plausible but only one I can think of which would take place DURING OTL war.
1. No Czech purchase/British blockade of coast
Prior to the 1948 war there was no Israel and so any arms purchases by the Yishuv were handled illegally and subject to British consifcation whereas the Arab states could purchase arms freely on the post WWII international market. While they used the opportunity poorly that still meant that the early phase of the war saw the manpower poor (in potential reserves that is, not drafted personnel) Israel forced to send infantry against Tanks and airplanes. Egypt also had total sea superiority, blockading the Israeli coast.
After the war started an international embargo of weapons and "war making Material" (including oil. WHich the Arabs of course already had and Israel didn't) on all combatants was imposed so Israel (or the Arabs) couldn't carry out legal purchases. Israel still had a clandestine weapon smuggeling ring but the big shot into it's war making capability came from a deal approved by Stalin and the Czech government unofficially which saw the Czechs dump the WWII Surplus of the Skoda factories for a pretty good price into the Israeli purchasers.
The products, especially the messerschmidts were pretty crappy but it still made a huge difference. If Stalin reaches an agreement regarding Palestine with the British (as he did for Greece) then the arms shipments to Israel will be choked off. Arguably, this could result in the Arabs not demanding the U.N impose ceasefires (which, contrary to their expectations, benifitted Israel far more than them), mobilizing more of their man-power, developing their own clandestine arms imports and so forth.
However, given how limited the gains the Arab armies made during the first phases of the war
A more plausible POD would be slightly before the war broke out:
1. British delay leaving the Mandate by a year. Or the UN or even the U.S takes on the mandate (both were considered OTL).
The period between 1945-1948 saw the Arab states gradually build up their armed forces under relatively free conditions while the Yishuv was more constrained in developing it's armed forces. Another year, especially if accompanied by low-grade civil war between the Arabs and Jews in the Mandate, Could see the Arab states put in more effort into mobilizing and lead to many more Arab gains once the international presence leaves the Mandate.
2. Jewish arms factory nabbed by the British.
some 70% of Jewish ammunition in the early phase of the war came from this underground facility
http://www.jewishvirtuallibrary.org/jsource/Society_&_Culture/ayalon.html
In 1946 (I think) a British troop train was bombed near the factory and the "Kibbutz"/ammo factory workers were ordered to rush to assist them to divert suspicion form the Kibbutz. As a result the British did not trash the Kibbutz the way they did the other Jewish communities surrounding the incident. Change that a bit and the factory could have been uncovered greatly reducing the war making capacity of Israel in the early crucial days of the war.
As to what would happen if Israel is defeated.... Well, it's fairly obvious in my opinion that "Palestine" would be split up rather than established as an independent state with The Lion's share going to Jordan and Egypt. Jordan would want a connection to the CoastWhereas Egypt would seek to extend it's portion of the Coastal plain as far North as possible.
The Christians in Lebanon would not want to keep much of the Gallile (as it would add miore Muslims) but lebanese Sunni would want exactly that for the opposite reasons. Syria might want to extend it's control as far West as Possible (including Haifa?) in order to engulf Lebanon and absorb it in the future.
The question is:
a. Whether that division would be stable or whether, in the absence of a Jewish state to act as a magnet for Arab Millitary attentions you would see a second round of fighting between the Winners (think second Balkan wars).
b. Whether native palestinian leaders would accept being dominated by Syria, Jordan and Egypt.
c. Whether revolutions would still overthrow the Egyptian and Syrian governments.
d. What happens to the remaining Jews in Palestine.
e. What happens to Jews in the Arab countries after israel is defeated.
f. How the USSR and the U.S enter into the fray.
My take is that the conservative monarchies in Jordan and Egypt are unlikely to come to blows immediately so a semi stable division is likely to endure for a few years. Jordan gets Jerusalem and the Central Coastal plain North of Netanya to Haifa. Egypt gains the Negev and the Costal plain to netanya. Syria Grabs almost all of the Gallille and ends up scuffeling with Jordan Over the Jerezal valley and with Lebanon over the Haifa Bay area.
Syria is likely to be the "Odd man out" as it's unstable republican Pan-Arab government is distrusted by the Egyptian and Jordanian monarchies (allied with the Hashemites in Iraq) and, of course, by Lebanon. but those monarchies cannot endure for long. In Iraq the Hasemis are too allien in Egypt King Farouq too corrupt and too out of tune with the national feeling in the army and in Jordan, the "Western territories" are much larger than OTL and inhabited by a far more sophisticated and less desperate people. They are going to want a share of power faster than king Abduallah and his Bedouin clique are willing to give.
the USSR will probably back Syria and the "new radicals" against the British backed Monarchies. The shape of the resulting conflicts depends on which Monarchial domino collapses first.
I think it will be Egypt. Later than OTL and possibly not led by Nasser but it will happen sometime in the mid 50s. OTl Syria proceed to combine with Egypt in the united Arab community rather quickly but the fact that they were separated by Israel prevented an overt takeover of Jordan or Lebanon (1958 civil war) and eventually Pan-Arab sentiment faded.
TTL Nasser, or whoever is running the show, may decide to remove a certain Bedouin monarch by main force- with the backing of his "Palestinian" subjects.
Whther the resulting Pan-Arab state would be stable, continue expanding (to Iraq, Libya, Sudan and the Gulf. Think the Magreb is too much in the 60s) or be a nice place to live.... well, the only OTL analogy in Nasser's invasion of Yemen (from which Sadat withdrew after the 1967 war).
http://en.wikipedia.org/wiki/North_Yemen_Civil_War
To put things in perspective, that conflict killed about twice as many people, and many, many more civillians than all of the Arab Israeli wars combined.
If the resulting PAn-Arab state IS stable then It would probably align with the USSR Vs the American backed Turkey, Saudi Arabia (which prevents Arab immigration form the North, preffering to import Pakistani workers instead) and Iran under the Shah and the French backed Mhagreb states.
Then the Iranian Islamic revolution happens pretty much on schedule and things get... interesting.
As to the Palestinian Jews- massacre and expulsion. King Abduallah suggested a Jewish autonomy under his protection but I can't really see that as having a chance. OTL every one of the 17,000 Jews in areas captured by the Arab armies was killed or expelled. The longer and harder the war is (POD #1 leads to the longest war.) the more immediate the massacres and expulsions are. An immediate victory with few casulties proabbly just means a lot of undirected attacks by local Palestinians and looting soldiers combined with officail opression that lead the Jews to leave La mode Pied-Noir over a period of a decade. The U.S may demand a temproary protectorate over survivors in Haifa and the coastal plain but I think the State department will veto it.
The Question is- where will they go? Stalin goes into an antisemitic Binge in the 1950s. the Central European states are not someplace that any Jew wishes to return to. And the West has closed it's gates to Jewish refugees post WWII (attitudes realy shifted only when the generation that grew up in WWII came to power- say the 1970s). So "never again" may well become "yet again". Unless American jews are confident enough to lobby for a rescue. If they do so openly however U.S antisemitism increases and you get a delay on American acceptance of Jews in society.
Arab state Jews- Again, OTL nasser gradually expelled the cypriot, Maltese, Greek and other Minorities from Egypt. Jews won't get as rough a treatment as they did OTL but they will still prefer to gradually leave (as North African Jews did when French rule ended). It will probably happen more slowly and less completely. Most will probably head for France and Britain. Would an early immigration of Jews, and relatively lower class Jews, to those countries sparked renewed antisemitism? probably, but more in the sense of delaying social acceptance than preventing it completely.