And come to think of it, I'm not familiar with any Xbox open beta programs by any other publishers, either.
There have been several. The only one I've ever participated in that actually operated like a normal beta was the one for Blur. Usually, Xbox betas are basically just demos, but that one actually had an active forum with discussions between testers and developers, and our input had a noticeable influence on the final product. I don't recall how
open it was, but it was very easy to get into and they were always e-mailing us codes to give out to friends. Then after a couple of months, they opened it up to everyone without requiring any kind of registration (unless you wanted to register for the forums, of course).