Barr vs. McCain
Earlier this year, I decided not to vote Libertarian for two reasons: 1) Libertarian votes would be interpreted as support for isolationism; 2) I thought the Wall-Street wing of the Republican Party (which McCain is currently allied with) was worth supporting against the populists.
The financial bailout made both of those dubious. Libertarian votes will now be interpreted as opposition to bailouts and the Wall-Street wing of the Republican Party looks much less worthy of support.
The deciding factor was the attempt by Obama supporters to get the votes of every conformist in the United States. This is partly a matter of the “Obamacons” and partly a matter of the possibly-manipulated opinion polls. I haven't seen this concentrated an attempt to make it looks like “everybody's doing it” since people were passing around joints in the 1970s. (In case anybody was wondering, I was in the control group during the drug-experimentation era.) A non-vote or a Barr vote would whisper that the conformists didn't succeed with me but a McCain vote would shout it. I plan to vote for McCain out of sheer orneriness.