My 3G did the same thing when I updated to 3.0. After I updated, I got an error message in iTunes saying something about my SIMM card not being valid or being bad, or something to that extent. All I could do with the phone was make emergency calls. I was crapping my pants, thinking something got screwed up (I didn't literally crap my pants, that was just a joke), so I called AT&T and then they transferred me to Apple. Then, while I was on the phone with Apple, I plugged the phone back in to my PC and then it magically started working. I think that something on Apple's end didn't sync properly. You've gotta remember, there are a zillion people also updating their phones, to Apple's servers are probably getting slammed, creating some difficulties.
Keep trying to plug your phone into your computer while iTunes is open, and hopefully it will eventually work. If not, call them. Then it will magically work like mine did. lol.
Good luck!