Unlike many libertarians, I'm a believer in voting for major party candidates. Voting is, of course, expressive behavior. On the other hand, so is not voting. Voting is basically a free rider/collective action problem, and if libertarians think these can be solved via private initiative, they have an obligation to demonstrate that it is so.
Besides, not voting seems like a way of trying to shuck responsibility for having a preference. Most of the libertarians I know who do not vote are for Obama, or at least, against McCain. But by not voting for him, they can disclaim responsibility for any results. The problem with voting for the winning candidate is that you can never see the counterfactual, so almost by definition, libertarians are going to end up regretting many of the results of their choice. If you pretend not to have had a choice, you don't have to admit that you willed, in some sense, the bad outcomes.
I'm not voting because I forgot to register. But that doesn't absolve me from whatever happens next, because I wanted Obama to win. I may not have effected the outcome, but I did believe it was preferable to the alternative. Now if he's even more of a cluster**** than I expect, I'll have to admit I was wrong.
(Of course, I can always say McCain would have been even worse, just as many disappointed Republicans argue that Kerry would have been even worse. And it's possible that they're right; until we invent inter-multiverse transport, we'll never know.)
All of which is a long way of saying that unless you really cannot generate any preference at all between McCain and Obama, you should probably vote. Yes, it's a pain in the ass. But that's civil society for you.