this may or may not have already been mentioned. i apologize in advance for not having the time to skim through all the comments as i'm still at work 🙂
for the past year i have been using an iphone 3G and this morning purchased a 3G S. they activated the new SIM card (that came with the 3G S) instead of recommending I just swap the old one with the new. as a result, i received the message: "waiting for activation. this may take some times." after an hour, a friend emailed me with the following recommendation to activate my phone and it worked immediately.
first, i turned off my old, 3G phone (don't ask me why i hadn't done this already). then, i power cycled the 3G S (turn off and then on again). within seconds the phone notified me that it was activated.
i realize this explanation is extremely simple and may only apply to people that are activating a new SIM card. this solution probably won't help people experiencing delays with itunes. but hey i thought i'd mention it in case someone is stuck in a similar situation.