Password is in the keychain, and is correct. From the timestamp, it isn't being updated when I have to retype it. Access control is SystemUIServer, Keychain Access and Internet Connect.
The problem seems intermittent - while the network connection never seems to come back automatically, sometimes I can count to ten, click on the airport icon in the status bar and reselct the wireless network, and it comes back to life. Other times, that doesn't work (it pops up an error box) and I have to select other etc. and re-enter the WEP key.
It still seems to be a WEP-related problem though, because if I turn of WEP on the router, then it automatically reacquires the network without intervention.
It's OS X 10.3.6, updated yesterday. Everything else has been updated too.