I have ATTWireless GoPhone, no-contract plan. The Phone pops with "Sim-not-ready". I do not have a clue if there is a menu item to choose or if it is not an option because I don't have a "contract"--Social Security # phobic.
I want to try a different service that is more user-friendly and doesn't cop $124 in 10 days. (see earlier thread-"Expensive lesson...")
An earlier tech report said the cell phone providers put some sort of "lock" that prevents using a phone on another service even if moving from a GSM to GSM or CDSM to CDSM or whatever the letters.
Dirty trick if they do. I don't need/want a newer fancy-schmacy $100+ phone again. I hope there is a work around.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic