It depends on what you mean by "unlocked" Defy, and which division of T-Mobile the phone will operate.

If you bought an unlocked "global" version of the Defy, and plan on using it in the States, then you will most likely not have 3G service on T-Mo (USA). Also, the phone's IMEI # may be in the system, rendering functionality on the $10/mo. "web2go" plan null.

However if you bought a T-Mo USA version Defy, unlocked it, it could be used with AT&T's full 3G service, (850 band) and qualify for the $10/mo. MediaNet plan.

If you plan on using an unlocked global Defy outside the States on T-Mo, then disregard this post.