T-Mobile's statement on Engadget is as follows:
While the Nokia Lumia 810 features a chipset, which provides the hardware capability to support both HSPA+42 and LTE, the device will not automatically access our LTE network and, at this time, it will not receive an update to do so.
T-Mobile is committed to enhancing customers' experience with our devices, including providing upgrades to device features and 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 810 when they are available.
THe handset still has access to HSPA+42 network, which offers a decent amount of speed, but owners looking for the ultimate speed (at least in today's market) are going to need to look at another handset.