If Obama had won any states with a plurality, you might have an argument. But in those states he won, he got at or over 50% in all but Missouri, which is yet to be determined at 49-49. So half of McCain's supporters could have voted for Mickey Mouse, for all it matters, the outcome wouldn't be any different than if McCain had gotten every last non-Obama vote.
Personally, I voted Libertarian. Obviously Barr had no chance (final tallies put him at about half a million votes nationwide), but I'm sick of tolerating "the lesser evil". McCain isn't quite so bad overall as Obama in my opinion, but he's bad all the same in different ways (the McCain-Feingold Incumbent Protection Act, for one). Considering the outcome was a forgone conclusion, there's no harm in sending a message to the parties that we're sick of candidates who are more concerned with protecting their own jobs than our rights.
With any luck, people will get their fill of Obama over the next four years and we'll have some better options next round. Small hope, but nothing's going to improve if everyone keeps tolerating the status quo.