I'm also getting this - I sold my ATV2 which worked flawlessly with my wireless network and got an ATV3 which steadfastly refuses to connect. If I connect via ethernet cable it's fine, If I take the cable out and put the ATV in the same room as the router then it works. If I move it to the room I actually want to use it in, it sees the network but when I try to connect with my password it throws up the same error as you.
Check the thread at https://discussions.apple.com/thread/4386679?tstart=0 There seems to be a problem with the time settings when the Apple TV can't connect to the network. That may be preventing your 802.1X settings from working.