I think the NATO response would depend on how Russia actually went about invading, and if it goes that far, their occupation strategy.
For example if Russia simply rolled into the Crimea and some of the eastern parts of Ukraine and stayed there it would be pretty bold but at the same time it would be relatively difficult for NATO countries to deploy effective force to the area. On the other hand a full invasion and occupation probably couldn't go unanswered (or NATO would lose its credibility), and there is an easier 'in' with an operation launching from Poland etc. Alternatively you have the scenario in Georgia where Russian forces engage in a relatively limited offensive and don't end up as occupiers per se, where no only were NATO strategic options limited, but there wasn't any obvious reaction that didn't involve going directly into Russia itself (ie. there wasn't an option to simply fight until Russian forces withdrew from occupied territory).
As a side issue, NATO is somewhat surprisingly not that popular in the Ukraine:
http://en.wikipedia.org/wiki/Ukraine%E2 ... in_Ukraine