The only difference I know of is the printing on the card itself, unless you have an early Cingular 3G SIM you can teather through Media Net and not have any additional charges. If you have a Cingy 3G card I would leave it as is. I had the opportunity to get a new ATT SIM when I got my Tilt a few weeks ago, but I declined. I'm still using my Cingular 3G SIM. -Jay