Even Balance Inc. have updated their PunkBuster Wolfenstein: Enemy Territory news with information on "Empty CDKey" messages.
Players who are getting "Empty CDKey" messages trying to connect to PunkBuster ET Servers may attempt a manual CDKey registration. Press the tilde key (the ~ key) to bring down the game console from any game screen and enter the following command (without quotes): "/pb_cdkeyreg". You should see a message stating that a Registration Request was sent to the GuidAuth Master Server. After a few seconds (perhaps up to 30 seconds) you should see a message stating that the CDKey Registration was successful.
After that, there should be no more messages about an Empty CDKey when trying to join a PB Server. Players having problems with this procedure (or any PB-related problems in ET) should email support@evenbalance.com and describe in as much detail as possible the problem encountered.