This article should state...

This article should state that while it is possible to join a network using 64-bit WEP, Airport Utility will select 128-bit WEP by default and will fail to configure the router properly. The only solution is to configure it manually. A better solution would be to use either WPA or WPA 2 encryption, but a lot of ISPs and router manufacturers seem to set up devices to use 64-bit WEP by default.