T-Mobile USA is in the process of testing UMA with a select set of consumers. The handset being used in the test is the Samsung t709, a handset that was announced this year at CES. UMA or Unlicensed Mobile Access is a way for phones to jump onto and off of a WiFi network and use Voice Over IP technology.
VoIP calls don't use the mobile carrier's network and are cheaper for consumers, so the idea is that when you're at home or the office and in range of a UMA WiFi router your calls will be of the cheaper variety. The challenge is switching between the mobile and WiFi network seamlessly--without dropping a call.
No word on when this could be a public product. It's also entirely possible that T-Mobile goes ahead and launches the t709 without UMA support.