I never could get my jailbroken/unlocked iphone 3g to activate facetime on t-mobile.

But my factory unlocked iphone 4 activated almost instantly (also on a t-mo pre-paid no data plan). I did it at home and probably had a wifi connection at the time, but supposedly the carrier has to send some sms messages back to the phone to complete the activation, unless they changed how the process works.

Did you take it in to the Apple Store?

-David

There are several workarounds to resolve that "waiting for activation" problem.
Happened to my iphone when i tried to use iMessage.

My suggestion is to search for that "waiting for activation" problem and see what people did to resolve it.

For mine, i had to set my phone info to myself in contacts.
Previously, the phone info was blank.
Once that happened, i had to enable Facetime and iMessage to get rid of the "waiting for activation"

Other people had other workarounds which solved their problem.