Im considering changing my mobile carrier over from T-Mobile to ATT so that i can get a new iPhone 3Gs but im wondering how easy or difficult it is transfer a cell number (or in our case 3 numbers) over to a new carrier?

I know in the UK its literally a case of just signing paperwork and within 24 hours the number has been changed over, does it work the same way in the US or is there more to it than that?

Regards,

Lee