There’s a way to avoid the standard AT&T contract plan and sign up for pre-paid minutes on your iPhone. With this hack, you can activate your phone for a fixed amount. Even after you’ve used your prepaid minutes — or remove the sim card entirely — the phone will continue to function as a WiFi enabled iPod.
The trick is to fail the credit check during the registration process. This will force the registration software to offer you the prepaid plan instead! It’s as simple as entering a bogus 999-99-9999 SSN during the registration process.
Prepaid iPhone hack @ TUAW – Link