In an official statement to TMoNews T-Mobile USA confirmed that they will not be releasing the WP 7.8 OS upgrade for the Nokia Lumia 710, writing:

"At this time, the Nokia Lumia 710 will not receive the software update. T-Mobile is committed to enhancing customers’ experience with our devices, including providing upgrades to the latest operating systems, and T-Mobile aims to deliver software upgrades when it will provide a better experience for our customers. We’ll share additional information on upgrades for the Nokia Lumia 710 when they are available."

A really sucky situation for anyone out there who picked up the Lumia 710 on T-Mobile in the past year. The statement does however suggest it is not a closed case, and that there may be further developments in the future.

Via WPC