You wont be able to use the UMA (wifi calling function) from what I understand that's carrier specific for the device, ie, you need to be on Rogers network to utilize it. I dont believe At&t even offers the feature on there phones anyway??
T-Mo will though upon the release of the 8900 in the US on Feb. 18th! Can't wait! ;)
Here's a thread with some discussion on the topic
http://www.blackberryforums.com/gene...-6-1-94-a.html