For an unknown reason, T-Mobile has decided to skip upgrading the Lumia 710 to the latest Windows Phone 7.8. In the United States, T-Mobile is the only carrier with the Lumia 710 device. Apparently, T-Mobile doesn't feel the upgrade to Windows Phone 7.8 provides a better experience for their customers.
At this time, the Nokia Lumia 710 will not receive the Windows Phone 7.8 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.
Windows Phone 7.8 has already started rolling out to other Nokia devices, including Lumia 710's that are unlocked or on other carriers. It is possible to manually upgrade your T-Mobile Lumia 710 to the Windows Phone 7.8 operating system by using this hack posted on XDA-Dev forums, though this is at your own risk.