11-16-2008 08:45 PM
I once bought a T-mobile prepay phone from a local store and threw my sim card from my T-mobile monthly plan and it worked just fine.
You can switch sim cards between any T-mobile branded phone as far as I have researched online. If it says another brand on it.. like cingular then cingular probably has the phone locked so ONLY cingular sim cards will work.
11-16-2008 09:11 PM
As the person above correctly stated, it will work. If you want data and the Sidekick you will need to call T-Mobile and ask for the Sidekick Data Package which will allow unlimited text, data, email, and IM.
If you do get a locked AT&T/Cingular phone you could always get an unlock code online (you will probably have to pay though).