Austria's a good probable. Finland is treaty obligated to allow passage to the Soviets, but whether they'd actually live up to said treaty obligation can be considerably debated... either way they'll get dragged in. Sweden's another probable, definitely on NATO's side. Yugoslavia's a "maybe, could go either side". China's another "maybe", although if it does join it almost certainly will on NATO's side.
Not
exactly true. This is the actual wording of the relevant portion of the Finno-Soviet Treaty of 1948 (my translation):
Article 1
In the case Finland or the USSR, through the Finnish territory, are subjected to an armed attack by Germany or one of its allied nations, Finland, as true to its commitments as an independent nation, will fight to repel this attack. In this case, Finland will commit all available resources to defend its territorial integrity on land, on the sea and in the air, and does this within the national borders of Finland, pursuant to the obligations defined in this agreement, assisted by the USSR or together with it if needed.
In the above-mentioned cases, the USSR will give Finland appropriate help, the giving of which will be agreed upon by the contracting parties.
Article 2
The high contracting parties will negotiate with each other in the case that the threat of an armed attack, as meant in Article 1, has been established.
So, you see: there is no mention of an obligation to allow passage to the Soviets, and legally, strictly speaking, Soviet troops could only enter the Finnish territory if invited by the Finnish government, by the Finns asking the USSR such military help in the appropriate negotiations as defined by the Treaty.
Now, in reality the USSR would in the case of war twist Finland's arm in many increasingly drastic ways to allow its forces access to the Finnish area, in an effort to safeguard Leningrad, Murmansk and Kola, and the area in between, and to get more airspace to try and stop expected inbound NATO bombers, etc. But if the Finnish government and military have any balls, they can draw out these negotiations and eventually, if they do not accept Soviet help, they would force the USSR to invade Finland unilaterally, outside the scope of the Agreement, which could of course mean war. Simply going by the existing agreements, the USSR can do nothing if the Finns pigheadedly refuse to ask them for help, except to back down or (more likely) make war on Finland.
If you recall, the above chain of events is essentially what takes place in my P&S spin-off. It would be more likely that the Finns would cave in under Soviet pressure, sooner or later, but still some negotiations would be necessary before the Soviet troops enter Finnish territory.