[youtube=http://www.youtube.com/watch?v=3KmfXupi9cg&] ...or AT&T.
Tmobile, in desperation to not become irrelevant, has done a substantial amount of work in the areas of expanding their network and trying to keep up with the Droid phones of Verizon.
If, however, a deal was in the works to bring the iPhone to TMobile (something I think they would definitely go for) they probably would not have released this commercial.
If you have ever used FaceTime, you know how easy it is and how well it works with a good wifi connection. You also know that when you get a weaker wifi connection or a slower one, the video quality and framerate becomes embarrassingly bad. The iPhone, too, has apps approved on the App store that allow a user to video call via 3G services. The quality is nothing in comparison to FaceTime. Not even close. Slow connections=poor experience.
Besides, every single carrier has moved away from the "unlimited" data model. If you think you have unlimited, you were either grandfathered in or there is a cap and you didn't read the fine print. It would not be necessary to remind you of how much data off of a 3G or 4G network the phone would use to video chat.
Waiting until the kinks can be worked out hasn't seemed to hurt Apple.