I don\'t recall what values are valid for SSIDs, but I do know that the Network Setup Applet has trouble with some chars, such as space.
To work around the problem, put anything in the field, then use an editor such as vi (you may need to su first to obtain write permission) to edit the file /home/etc/pcmcia/wlan-ng.opts. Put the correct SSID value in the \"DesiredSSID\" string.