I take everything my baggage allowance permits. Not because I'm scared of breakdown, but because holidays are my most important and least repeatable regular photography opportunities, and I like to mix scenery, wildlife, people and whatever else is worth making images of. So I take whatever I can carry, and then choose day-by-day what gear to go out with, leaving the rest in hotel or apartment.
Matt: very wise words. Unfortunately, in these wonderful days of "airline security", I find "whatever I can carry" can be remarkably limiting.
A cautionary tale, however:
Last year (ie. June 2007) I and my father went on a wildlife photography course organised through
Australian Geographic magazine. I took a 2nd
near everything. Lots of people (especially including Dad) pointed and laughed. Too much gear! 'Till Dad dropped his camera, with external flash on, and ripped the hot-shoe off. He took over my 2nd camera. Someone else's card reader failed. I had a spare, which I loaned them. Another had a shutter failure - but I had a film body as "2nd spare", so they could use that. I had hard-disk space (external), film (b&w and colour), spare USB, network and telephone cables, etc. etc. (most of which, one way or another, someone used).
None of which could have been found within a 200k (120mile) drive (and way worse in terms of road conditions). OK - it didn't help
me because I had nothing fail or break (mostly by good luck rather than good management) - but I was amazed at how many things
did go wrong. And also amazed that nobody but me had a backup plan ...
...Mike
(P.S. my usual and more "general" travel backup plan is an Olympus mju-II aka Stylus Epic which cost almost nothing, takes nearly no space and still takes a good photo. But for "one off" trips like the photo course / rainforest trip I do a little more.)