How To Use An iPhone Without AT&T Service

You can't exactly use an iPhone to make calls without AT&T Wireless service but you can use the iPhone simply as a WiFi Browser and iPod without it.

Erica at TUAW reports that if the SIM is removed and replaced with an unvalid/unactivated SIM card or if remove the SIM entirely WiFi and iTunes syncing still works. You can also use an activated SIM from an AT&T Pay As You Go mobile phone.

If you want to use an AT&T prepay account instead of a plan you can simply enter an invalid Social Security number during activation.