The WP7.8 update which was sort of console 1st gen. Lumia users (Lumia 710, 800 and 900) will not be available for T-Mobile's Lumia 710.
T-Mobile confirmed that the WP7.8 OS upgrade will not be coming to its Lumia 710 users.
Here is the statement:
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.
If you own a Lumia 710, you can still manually get install the update, just follow this Hacker guide.
This isn't the first time T-Mobile has screwed its Nokia users by not delivering updates, remember when the Symbian Belle update? the Nokia Astound (C7) was also denied the update by T-Mobile.
I do not know what T-Mobile's game is but it's not pretty, if they do this one more time, they will lose their Nokia users (all 50 of them). Because let's face it, I don't think t-Mobile has enough Nokia Lumia 710 users to care about rolling out the update.