It doesn't look like the iPad works with the "wide channels" feature of Apples 11n base stations. I have an Airport Extreme setup at 5 ghz with wide channels, my MB Pro and iMac work fine, but the iPad gives me an error when trying to connect.
iPods, iPhones, and iPads all only support 802.11b/g/ speeds. They cannot work with 802.11n protocols. You will need to configure your AEBS to us 802.11n and 802.11b/g/ compatibility otherwise you will not be able to connect to your local wireless network. Unfortunately, this means the router has to drop down to 2.4 GHz so when in 802.11n mode you cannot connect to any "n-only" channels nor use the 5 GHz speed.