If it gets released on T-Mobile US, I would assume (hope) they will include the UMA feature. This is an out of box feature that automatically uses Wifi for your cell phone calls. I have had it on my T-Mo BlackBerry for about the last 2 years and it works fantastically. Best part is calls over Wifi don't count towards your plan minutes.

While I am at home, in my office, all my calls go through Wifi, then when I hit the road it automatically switches to cellular. It will even auto switch from Wifi to Cellular if I am actually on the call when walking out the door.

The sound quality is great and I have never had any issues. It will even pick up T-Mobile HotSpots and if the signal is better it will automatically use the Wifi over the cellular, e.g. at the airport or any other HotSpot.