"In response to your concern, we understand that you want to confirm if the Windows Phone 7.8 will be available for Nokia Lumia 710 from T-Mobile. We are sorry to inform you that there are a small number of operators that have chosen not to offer the update to their users.
It is important to recognize that this decision was made solely by the operator and these include the T-Mobile network in the United States."
Email reply from Nokia. So we'll see.
I have T-Mobile Nokia Lumia 710 (no contract) which I got unlocked from T-Mobile 5 months back.