I picked up a new Netgear MR814v2 wireless router this weekend and an Airport Extreme card for the iMac. I set the router up with 128 bit WEP and my PC had no problem, but the iMac refused to connect, no matter how I configured the WEP key. It even failed with WEP disabled entirely.
After fighting it for a few hours I found the fix -- upgrade the router to version 5.0.1, and the Airport card will sync up happily with or without WEP.
I hope this saves someone some time!
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030920233746442