Like Duvi said, T-Mobile's always had a good relationship with Nokia. Sounds to me like Nokia went, "Hey, T-Mo, you want to sell the 710? We have the production to make that happen. Can't do the 800, though." and then T-Mo said: "Sure, why not?"
This is a good move. There is a market for low-end smartphones. There are plenty of people who can't shell out $200 for an iPhone or what not. If this comes out at, say, $50 or maybe free, it will be great. The more people get introduced to Windows Phone, the better. Eventually those low-enders will "graduate" and buy higher-end Windows Phones when they come out.