From Apple’s developer site:

We want to give Apple developers the chance to build and test their apps on the new Apple TV before it becomes publicly available to customers later this fall. Register by September 11, 12:00 p.m. PDT for an opportunity to order an Apple TV Developer Kit. Availability and supplies are limited.

If you’re a developer who’s even a little bit interested in building apps for the Apple TV, I’d suggest registering.