I only took up riding again fairly recently, so it was quite a surprise to suddenly have an attack of vertigo, phobia, whatever you want to call it, when attempting to go over a couple of bridges and overpasses a few days ago. I got off and walked the bike across. There was no real danger, of course -- I just suddenly got that sinking "What if I fall" feeling and decided that if I tried to macho my way through it I might get dizzy for real.
I've always had a moderate fear of heights, but I wasn't expecting it to bite me in the bicycle saddle.
What about you? Any phobias that affect where/when/how you ride?