I've been experiencing this problem since I first unboxed my IPad 2. I returned my first IPad 2 within a week because of the the intermittent wifi problem. The second IPad 2 did not experience anny connectivity problems, but I had to return this unit because it developed stuck pixels (so frustrating!), and the 3rd IPad 2 - the one I'm using now - has been hit or miss with connectivity. I think I may have bypassed my problem for the time being though. Here's how ...
Up until recently I was using the westell 7500 modem/router given to verizon/frontier customers. This router has it's fair share of issues. The IPad 2 would occasionally struggle to retain a connection, but the problem would hang around for a day or two then the IPad would function normally.
Settings - security status open (IPad didn't like WEP or WPA on this router ... would not connect) - wireless devices filtered by mac address only
I tried removing all security, including mac address filtering, using different encryption settings, but nothing would alleviate the problem. Recently my dedicated ereader had a firmware update that made it icompatible with the westell 7500, and regardless of the settings I could not connect my ereader to the router. So at this point I dug out an old Netgear wndr3400 and let it handle all of the routing reponsibilities. Wouldn't you know it both the ereader and the IPad 2 connected immediately. I can only hope that the IPad 2 remains stable on this "new" connection, but I'll update with any information if it doesn't.
This is not a perfect solution, and may very well only address a specific connectivity issue with the westell 7500. I would recommend to anyone out there using this router to disable wifi and use it strictly as a modem, and virtually any other router out there offers more features with greater ease of use.