(yeah sure you prob want to back up to somewhere off your phone)
It also "fixes" a lot of potential "problems" like people running p2p software on their phones or using VoIP instead of the much higher priced voice calls by the phone proivder (yes. voip works today, but it could work ever so much better if these devices were accessible directly)
Edit: Apparently it is possible. (1)
But I guess that would only be valuable if they also gave you a static IP. And static IPs for client devices like phones aren't exactly a good thing from a privacy perspective. Ideally you'd have both a dynamic and a static IP, and new outgoing connections would originate from the dynamic one.
Why? That's what (Dynamic) DNS is for.
Could it be regional... T-Mobile enabling ipv6 for some cities and not others? Or perhaps the current tmobile firmware (mine is 2.3.6, T989, UVKL1) for their sgh989 disables ipv6 for the gsm/utms portion of the radio.