Can you use a T-mobile SIM card in a T-mobile prepaid phone?

Yes. if it is a tmobile sim card and a tmobile phone then this will definitely work. you will also be able to carry on with the same price plan and number you used before. If you currently have a Tmobile phone on a contract plan that is complete you can also just change your plan to a pre paid plan and keep on using the phone that you have.

UPDATE: As of April 2012, TMobile prepaid phones may only connect to T Mobile towers, limiting your signal (since they also use ATT towers) and giving you varied signal strength depending on your location. So, it's best to unlock any prepaid phone. Ask T Mobile to send you the unlock code or pay someone to unlock your phone.