That is something I've been wondering, if it does come to war... will the war revolve entirely around Crimea, or will Russia invade Ukraine on all fronts. If they do, I would expect the other nations in the UN to send in some troops to try and 'hold the line' and force a stalemate until there can be some agreement talked out. I certainly hope it would not come to something akin to WW2 of invading any country that is stepping in and end up with armies landing in mainland Russia and a full scale war. It really shouldn't do, as none of the countries I could see getting involved (US, UK etc) are exactly nearby.
Reading wiki to try and learn some more about the backgrounds, it mentions that up until the recent revolution there was no official language for the Ukraine, there has since been declared one, which is part of the cause of the Russian unhappiness. It also mentions that in the past Stalin epelled all non-native (Bulgarian, Tatar etc) from the region and they have only in the past 20 years or so been able to re-settle, which I guess leads onto what Black Wizard said about them not supporting Russia in this.