See this thread - I didn't post it, but these are my exact symptoms too, since today:
http://discussions.apple.com/thread.jspa?messageID=8411431
It's only just started to happen, happens on all (even fresh) accounts, and isn't fixed by a reinstall and/or trashing the preferences.
WTF MAN, WTF?
I don't hold out much hope for the Apple discussion to solve it, but you guys rock. Can you fix it?
EDIT: never mind, already sorted.... tsk!