BAD APPLE
I had the same problem. I was stupid to upgrade my atv2 and the airplay icon dissapered after 1.5 hours. Rebooted the router and it worked for another 1.5 hour. I was going crazy! After 1 year of perfect airstreaming and now this.
This seemed to work: (going in my third hour now with airplay)
I had my 5GHz wifi-connection on atv2 and macbook air. 2.5 GHz on my iPhone.
Changed atv2 and macbook to 2.5GHz and ts still running.
This is stupid because I want the 5GHz for all my videowatching!
I hope Apple solves this and I will upgrade once more then NEVER again!
Cheers!