At home, I have never been able to connect my iPad 2 or AppleTV to my 5 Ghz WiFi network. I can see my 5 Ghz wireless access point ID (SSID) listed and do get prompted to enter my password, but the result is a cryptic message "Unable to Join Network" with no additional details.
"To allow your iPad, iPad 2, and AppleTV to connect to your 5 Ghz WiFi - if you have set your router set to '802.11N Only' mode - turn on your WMM setting (in the QoS section). Or change your 5 Ghz SSID to use 'Mixed Mode' instead of "802.11N Only' mode."
Finally I found a solution thanks to this post in the Apple discussion forums. To summarize, to optimize my home wireless I have my 2.4 Ghz SSID set to mixed mode to allow my devices which don't support 802.11N to connect to my WiFi using 802.11G. On my 5 Ghz SSID, because I do not have any 802.11A devices, I have it set to use '802.11N Only'.
I had off and on tried to find a solution and the best answer I seemed to come up with were some obscure posts saying that the iPad/iPad2/AppleTV do not support 40 Mhz Channel Width on the 5 Ghz band... However changing between 20 Mhz Only, 40 Mhz Only or "Auto Select" had no effect for my iDevices, but it did slow down my WiFi speed on my laptop.
I found the solution to this connection issue surprising. There are two options I have confirmed to work. The first solution was to change my 5 Ghz SSID setting from "N Only" mode to "Mixed Mode" and restart. My iDevices connected no problem with "40 Mhz Channel Width" selection. Digging a little deeper the better solution (for me) was to keep the "N Only" setting but in my Quality of Service tab of my router settings turn on WMM support.