This is how I activated mine.
I started with a fresh iPhone 4. Activated with my friend AT&T card than I jailbroke it and unlock.
Step 1. Insert ur tmobile card.
Steep 2. Install ios4 mms fix from cydia( add repo xxxxxxxxxxxxxx)
Steep 3 reboot your phone than go to setting general=>network=>cellular network and make sure mms apn is set to wap.voicestream.com also Internet apn to Internet2.voicestream.com than reboot again.
Steep 4 make sure u set auto lock on general to never and connect to a wifi network.
Than go to general phone and turn on FaceTime.
A windows will come up stating that you will be charged. It's only 20c I think for international tx.
Leave the phone like that for 1 hr make sure you don't lock your phone and the phone is connected to wifi.
This is how I activated 2 tmobile iPhone 4.
I hope this works for you as well as it worked for me.
You can see te pic I have posted on my twitter.