Two different Dell Laptops, a Latitude D820 and an E5510 both running Ubuntu 11.10. Both will connect to wireless, but immediately drop the connection and then make me reinput the wireless key. The wireless router is a Cisco WRVS4410 using WPA-Personal security and is working fine for several windows laptops, Kindle Fires, Wii, Xbox360 etc. Its just these two Ubuntu laptops that won't stay connected.
Are you using network manager? If so, try using wicd. (I literally just had this problem 2 days ago, and this is how I solved it.)
I think network manager struggles with WPA for some reason.