Details here: https://sites.google.com/site/tmoipv6/lg-mytouch
You have to set up a IPv6 (-only?) APN.
Most interesting part: "T-Mobile's IPv6 service is an IPv6-only service that uses NAT64 and DNS64 in the network to connect IPv6 mobile users with IPv4 content. ". So your phone *only* uses IPv6 (no IPv4/IPv6 dual stack), and T-Mobile converts that to IPv4 when needed.
This method is probably in line with the current APN, which does NAT44. Going to NAT64 is then probably a small step, and easier than providing dual stack.