I'm having the same problem, it's a pain. I'm trying to sync through MobileMe.
When I just tried to sync my PC manually through the icon next to the clock, it said "cancelling" straight away. Last successful update was yesterday, but I know I'm having problems with it.
Now when I've gone into my sys prefs to have a look at my sync preferences, it's not there. My iPhone is syncing with MobileMe no problems, but my MacBook Pro isn't. Hmmmm, will call Apple and report back.