While poking around the internet trying to make sense of what is going on in Ukraine and why Russia is so interested in it, I now have something that at least makes sense. An interesting place to start is this South Stream pipeline that Russia is building in partnership with several western European countries (and apparently no US companies).
Reading thru this article, I came across this graphic:
(Click to enlarge)
The terminus where the pipeline leaves Russia is at Beregovaya, but upstream from that the only access is via eastern Ukraine. If an unfriendly Ukraine shut down that portion of the pipeline, there is no backup route. So that seems like a decent motive for Russia to maintain a presence in eastern Ukraine.
Of late the pro-Russian separatists have reportedly been losing ground, though the pipeline area still seems to be in the area of pro-Russian control. The pipeline isnt set to be operational til 2015, so maybe this battle for control will be going on for awhile.