It's certainly been a long time coming, but T-Mobile has finally begun rolling out WiFi calling to Android. The first device to get the ability is the G2, which was conspicuously absent from T-Mobile's press release announcing that WiFi calling was coming to the platform. This update has been hitting select G2s OTA and, along with WiFi calling, also enables native tethering, an update to the radio, and a handful of bug fixes. As with most OTA upgrades, it's taking some time for all users to get the notification, so don't be worried if you haven't gotten the call just yet.
WiFi calling for Android has been oft-requested ever since the G1 was introduced, especially since it was T-Mobile that initially offered Android. The only downside to this news is that we aren't sure just which Android phones will end up getting the ability to make calls over WiFi, with T-Mo only saying that the feature "anticipated to be available on a growing selection of T-Mobile's Android-powered smartphones in the coming months." While newer handsets are likely to be included, things aren't looking good for some of T-Mobile's older Android phones. Perhaps the industrious Android hacking community can work its magic to bring WiFi calling to all?
UPDATE: T-Mobile has just put out an official statement on this situation, and in it the carrier doesn't really confirm or deny the update's existence. This is what T-Mo had to say:
Wi-Fi Calling and tethering/Wi-Fi sharing are not currently supported on the T-Mobile G2. T-Mobile knows these features are important to consumers and we’re working to deliver them to G2 users in the future. We have nothing further to announce at this time.