http://www.theregister.co.uk/2007/07...obile_bt_deal/
T-Mobile and BT have struck a deal so that BT will manage the interconnections between T-Mobile's base stations and it's core network.
Initially, they will use leased lines to connect the base stations, but, ultimately, as BT switch on 21CN, they will be upgrading the base stations to connect to the network via Ethernet. This will give them greater bandwidth which they can use for streaming media, and various downloads.
Obviously, we, the consumer, won't see much, if any advantage initially, but it will be interesting to see what services they offer.