I restored my iPod Touch 4th Gen this morning and it activated fine, in fact it did it within seconds, the fastest activation of the betas I've seen. Maybe after so many people activated, it's goten weird, but it worked for my at about 1:05 PM PT.
You can downgrade if you were on a developer beta, but on a general public release, your right, you can't. (Unless you jailbreak your iOS device and save its SHSH blobs)
I had the same activation problem. I kept hitting the home button at the activation error screen and chose "start over". My phone just accepted the activation a few minutes ago. You will have to be patient if you get this error. Don't downgrade back to 5.1.1.
Your UDID will eventually catch up on Apple's servers. My phone was "bricked" for a few hours.