T-Mobile is one of the four major carriers in the US, even if it is number four. They have plans to further continue the tactic they have used all along, compete with lower prices. Their goal is to have multiple Android Smartphones costing less then $100. They do offer the LG Optimus T for under $100, but it is hard to see them having high-end devices reach that mark. The phones are already largely carrier-subsidized as it is.
T-Mobile USA CEO Phillip Humm expressed to The Wall Street Journal that he wants to bring down the price of all Android phones on the market. Other carriers such as Verizon, which just announced the Apple iPhone, would not be willing to join them. It is a method to gain more market share, however I don't foresee them rising out of the fourth spot in the near future.