I've been experiencing this behavior since 1.0 betas. Each time I relogin, quicktime is screwed — there's white backgrounds instead of videos in QT Player, QT browser plugin displays the QT logo while downloading the file and then draws a "?" sign instead of displaying the player, etc (iTunes plays music correctly, though). However, if I open Perian prefpane and remove/reinstall it, everything starts to work correctly (though it's a pain in the ass to relaunch all processes using Perian). My account is filevaulted, if it matters, and I'm willing to provide any necessary information.
