My Tungsten C is working at my work with WEP enabled. I'm not sure how to help you though...
What I did was enter my password, checked to make SURE it was entered correctly

, then cycled through the list of keys until it would take it (you know - the key has to match the length of the password blah-blah-blah) and try to connect.
Since there were only a few combinations, I found it pretty easily. Also, if you have total control over the airport, you could log in and MAKE SURE you have the right password by re-entering it.
Good luck! It really works like a dream; extremely fast.