Despite Apple using a new SIM card in the iPhone 5, T-Mobile users will looking to use the iPhone 5 on T-Mobile network will not be affected. T-Mobile USA responded to iPhone customers that they will be releasing the nano SIM card by mid October.
From T-Mobile’s Twitter:
We will be releasing the nano SIM mid October, stay tuned as we work to make your iPhone experience seamless as possible^JQ
Future T-Mobile iPhone 5 customers will need an unlocked iPhone in order for it to be used on T-Mobile network. This phone on T-Mobile, unfortunately, will be limited to T-Mobile 2G (EDGE) network. If you’re lucky enough, T-Mobile is rolling out 4G HSPA+ service across the country, which will allow you to use T-Mobile 4G on your iPhone 5. This service is currently available in Seattle, Las Vegas, and New York City.