This Ukrainian siege is beginning to make more sense to me. As other posters have noted, it's all about oil.
Found the following articles from BBCNews very interesting. The first summarizes the EU's oil/natural gas situation in relation to Russia and the Ukraine very well. It's straight-forward and to the point. The second article is actually a timeline of events concerning the Ukraine and how things have come to where they are since this country broke from the Soviet Union. Puts a lot into perspective.
BBC News - Ukraine crisis: Europe's stored gas high as prices soar
BBC News - Ukraine crisis timeline (Note: Timeline is in ascending order (most recent to past))