While T-Mobile's G2 may support calling over WiFi, according to TmoNews, it won't support one of stock FroYo's best features: turning the phone into a portable WiFi hotspot.
T-Mobile's official statement on the matter?
“T-Mobile does not currently support handset tethering or offer a tethering rate plan. Though tethering and Wi-Fi sharing will not be initially supported on the T-Mobile G2, we know that consumers are interested in these features and we are working to develop a solution to support them in the future.”
A flawed explanation at best, especially since the Nexus One, which runs on T-Mobile's network (unless you've got the AT&T version of the phone), supports the WiFi hotspot feature.