My brother-in-law Jason, like me, is politically conservative. Like me, he's not very impressed with John McCain.
Jason told me recently that he will likely vote third party for these reasons: he doesn't see much of a difference between McCain and Hillary Clinton/Barack Obama and he'd rather vote for a candidate that better reflects his own views.
At present, I am still leaning towards McCain. However, it would be that lesser-of-two-evils thing. I keep thinking that in the future I could certainly go third party. Perhaps I'll do it if I see a truly viable candidate, or I might do it to show my disgust at the GOP continuing to ignore their conservative base.
Interesting note: Michael Reagan, the son of Ronald Reagan, bills himself on his radio show as "The very independent Michael Reagan." Mike (one of the very best talk show hosts in terms of content) said he left the GOP because it's no longer the party of his father.
As for my wife, the lovely Jennifer, I've told her that if she's voting for the candidate she feels is the best one, she has nothing to be ashamed about--as long as it's not Barack or Hillary (or, as Jay Leno calls them, The Pleaser and The Freezer).