Most of the sites I'm based around don't have routers which broadcast the SSID. It seems that the Macs are incapable of re-attaching automatically to SSID nonbroadcasting routers. Is this an acknowledged problem or is there a switch?
I do not broadcast my SSID and my MB connects without issues. I had to enter the SSID manually when I setup the connection, but since then, it's completely transparent.
FWIW I've had some trouble having my keychain "forgetting" (or not being able to access) my WEP key after some updates. I've had to remove the entry completely and save it anew about 3 times in the past year. You might try removing the entry and creating a new one as part of the debugging.