T-Mobile were the first US network to carry Nokia's Windows Phone handsets, and while Windows Phone 7.8 is designed to extend the life of the first and second generation handsets, we're sure that deep in the bowls of T-Mobile USA there's a computer program that has worked out it's better for the company to gain new users with the latest handsets and churn the older handset subscribers than it is to support them with the 'investment' in testing the OS update.
From the quote given to TmoNews:
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.
In essence, you're not getting the update, but we're committed to providing updates... which is almost Orwellian in double-speak.
If customers are feeling brave, then they have the unofficial option of doing a manual installation - a procedure which is down at your own risk, and is not supported by the networks (it'll likely invalidate your warranty as well). There is also a 'one click' upgrader available from Windows Phone Hacker, but again, the risk is all your own.