I would like to share my experience, it might help others. I had ATV 1 and ATV 2 connected via wireless and Airport base stations with no problems or issues. Updated all to the latest versions, including my main computer using OS X Lion. After the last Airport update, both the Utility and the firmware, i received my two new ATV 3s.
Then the problems started. I found every day, several times, the problem of missing the ATVs in Airplay and not being able to load the libraries. The obnoxious spinning wheel on the ATV and the never ending "loading XXX library" message.
After reading several posts, resetting airport, itunes, atv, turning on and off home sharing, the problem did not disappear.
I decided then to change the Share name of my iMac, change the name of the iTunes shared library and choose custom names for both ATV 3s. Reset everything and it seems to be working, while i have not done extensive testing yet.
It seems that, for some obscure reason, the association between the names of these objects, bonjour, wifi and wireline setting, got corrupted. Seems to me that some ARP or reverse lookup was not completely working.
Since this was so frustratring, i want to share so others can elaborate as well.
Good luck.