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.
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).
