I had the same issue. The phone number it had was my sisters (Family Plan), and since I did not want to activate it and deactivate her number, I went to my nearest AT&T store to see if they could give me a new SIM card. When the lady tried to activate it with the everything straightened out she got this message in iTunes:
"The SIM Card in your iPhone does not match the SIM Card associated with your account. To activate your iPhone with this account, please use the SIM Card associated with your account. Please disconnect your iPhone, insert the SIM card provided with your iPhone, and try again. You may disconnect your iPhone now."
We tried activating with both SIM cards. She called Apple; and after waiting forever to talk to an Engineer, they told me to go home and Apple will call you. I have no phone because my 2G was deactivated when she went through the process of trying to figure this out. I have a 2G "ipod touch" and an inactivated 3G S. I have been waiting for a call for over 5 hours.