Could you refresh my memory as to when this was?
http://en.wikipedia.org/wiki/Treaty_of_Brest-Litovsk
Going by this, it would have to be some time late in December and require a much more pragmatic and stable Bolshevik party at the time. Assuming it all works out:
It saves Germany from invading Ukraine and dealing with the mess there. It also prevents the waste of manpower and effort in a singularly unproductive endeavor, forcing the Austrians and Germans to release manpower to work in their own fields (the Habsburgs will have a mixed reaction to this, because large numbers of ex-soldiers also turned to brigandry to survive the last year of the war). But it also helps them by not having to run trains out to the Caucasus area and conserving precious coal and limited locomotive resources.
This means that food actually produced can now be transported where it needs to go and the Austrians have only to focus on one main front (not having to maintain a presence in Ukraine, but still having Macedonia and Romania to deal with). The issue of annexations in the East is going to be hotly debated now that the spoils are limited to Poland and Lithuania, but this could help in Austria, as they are not now going to give Galician land to the Ukrainians for food, which stabilizes their political situation. It also means that the treaty of Bucharest happens earlier, which further frees up resources sooner for Austria-Hungary/Germany (not many though).
Germany is now going to have a tighter ring to hold, which means there will be a surplus of 4th-rate divisions left over. They could very well appear on an Austro-Hungarian front, but also be demobilized for labor.
But overall, I don't know if this much changes the situation in the West. Italy is still at war with Austria-Hungary, which is draining the Habsburgs to death. The Germans are still locked in battle in the West and won't be able to launch their offensives any sooner. In fact, this really frees up no useful troops for the March offensives. Really, the only thing I can think this might do is make Ludendorff more flexible (minutely) in negotiating. Without the promise of Ukrainian grain there is no illusion that Germany can just wait for food to come rolling in.
The battlefield situation remains mostly unchanged for everyone, as the troops involved in occupying Russia OTL were basically of no use for anything but standing around. They were not meant to fight at all and were equipped as such. Sure it might free up more artillery (very little) or machine guns (again, not many), but overall the manpower is not useful for tipping the balance just anywhere.
The war is likely to end about the same, but with the internal situations in the CP states more stable and the Russian Civil War changed in ways I cannot comment on. The only difference I could see being made is somewhere in the Balkans or on the Italian front where there are more German troops made available for operations. I doubt that many resources will be wasted in Macedonia, which was considered a sideshow to tie up entente resources.
Italy is the likely bet for the next joint operation, perhaps in conjunction with Kaiserschlacht. The austrians did this themselves in June with predictable results. There were other attacks in between the last offensive and Caporetto, but were a failure even before German troops were removed in December 1917. Now they might stay if further troops are freed up from the East. I don't have high hopes for their success though. Even if the Italians are forced to give way, the maximum number of Entente divisions are already present on the front. Their absence didn't make a enough of a difference OTL to allow the Germans to win, though they were withdrawn and returned as a result without taken part in the fighting IIRC.
At this point, there is just about nothing that will knock Italy from the war. They could be damaged enough to make their November advance unsustainable, allowing the Germans to occupy the Austrian parts of their ally and trying to hold that front once the line is reconstituted in the Alps, but I have my doubts.