T-Mobile confirmed that it will begin selling the iPhone 5S and 5C on September 20th, with pre-orders accepted on the 13th. This is the first time since T-Mobile started carrying the iPhone that it's been available at the same time as other carriers. If you're interested in getting your phone from T-Mobile, you can sign up now for a notification once they start taking orders.
T-Mobile began carrying the iPhone 5 earlier this year and all indications are it's been quite successful for them. T-Mobile added 688,000 subscribers for the second quarter, on the strength of both the iPhone 5 and the company's "Un-Carrier" plan, which it purports to be less expensive than the competition's: the contract-free plan starts at a base rate of $50 per month, for unlimited text and talk with 500MB of data.
T-Mobile noted in its announcement that it will unveil the latest changes to its program, which it calls "Un-Carrier 3.0," this fall.