Apple TV Borked...
I attempted to load the iOS 5 beta 3 code on my AppleTV today - the load went well, and then when I connected it to my television, I kept getting the message that it was unable to activate.
After playing with it for about an hour, I decided to "fall back" to the latest publicly released version, per the instructions provided on the developers' site.
Now I get a 1604 error and it won't work with iOS 5 beta 3 and it won't let me roll it back... as pi$$ed as I am about this, I also wanted to warn any of you considering making the move... DON'T DO IT.
And FWIW, the developer forums are worthless when it comes to assistance with restoring. I only decided to "go for it" after I read in the instructions that it could be rolled back. So until iOS 5 goes GA, one of my Apple TV units is a paperweight.