I have been troubleshooting completely inoperative wifi on my new ipad2 since Friday 3/11. Router is 5 year old Dlink DGL-4300 "Super G" (Atheros based) that has been solid with every other wifi computer device in the house. I was just about to give up and buy a new router, but I finally got it working last night.
The first thing I did was disable the Super G "Turbo" mode and switch from CH 6 to CH 11. That helped a little and made the ipad2 wifi barely work for just a few minutes one time. I then discovered that the ipad2 would connect solidly by disabling WPA2 encryption, but would refuse to connect as soon as WPA2 was re-enabled. Now the problem was definitely isolated to the WPA2 encryption between my router and the ipad2.
During the past three days of troubleshooting I ran across a posting in one of the forums about someone having ipad wifi encryption problems, and they were able to fix it by increasing the passkey length. I had been using a passkey phrase of only 10 chars, I tried raising it to 20 chars, but was doubtful that was it. Well BAM! The next time I went to connect the ipad2 connected right up within about 2 seconds and stayed connected solid and has been solid ever since.
If you are still having wifi connection problems
using WPA2, definitely try using a longer passkey of at least 20 characters. And on older atheros based routers, be sure to disable the Super G or "Turbo" mode because it does not play well with the broadcom based chipset in the ipad2.
HW