Though every Xbox One has the potential to become a development kit, that feature won't be available on day one, like seemingly many of the Xbox One's features (third party headset support, the ability to use the controller with PCs, etc). Speaking to Edge Online in a comprehensive interview worth reading in full, Microsoft Studios’ partner creative director Ken Lobb explained the delay, vaguely explaining, "that there’s some work we had to finish on the back end before we could enable it."
Still, it's going to be a potentially great feature whenever it happens. Lobb went on to explain a cool use of the dev kit ability in balancing future character additions to the new, sort-of-free Killer Instinct.
Using the can-do fighting game community as a resource to balance a character before they're released to the open market is a pretty neat idea. It's like a more controlled beta, in some ways, and not just one of those typical server stress test/marketing hype betas. Do you want to help balance the new Killer Instinct.
Xbox One: the state of play, according to Microsoft [Edge Online]