I agree, but people who are yelling at rude people often don't know how to do that effectively. While I may find the flash photography annoying, it's not nearly as bad as the guy two cars back who yells, "Turn off the @#$(%&^*# flash!" while my 5YO nephew is sitting in the car with me. Frankly, I'd much rather see the flash.
If you feel as though the rudeness (flash photography, cellphone light, talking, PDA, etc) warrants it, then yes -- do say something. But don't try to fix rudeness by being rude in return. I generally look at the offender (if I can see them), make eye contact, and say, "Please stop that." I don't give them a reason why, I don't make it a question to which they can answer "no" -- I state it as a firm request. Usually, it works. When it doesn't, I might make a second request. If that doesn't work, I drop it. Because in the long run, if the worst thing that happens to me in a 24 hour period is that someone used their flash on a dark ride at Disney World, then my life is going pretty good, y'know?
PS. To the OP -- did anything GOOD happen to you on your vacation? I've read all sorts of threads started by you about things that annoyed you (people stopping on the side of the road for pictures, flash photos on rides, etc.). I haven't read any threads that talk about you enjoying yourself!