Okay, as of ~4th October, EvenBalance (E and Activision no longer have a contract for PunkBuster support on Wolfenstein: Enemy Territory, and thus they have dropped support for it.
You will no longer be issued ETKeys by EB, and thus you will no longer be able to get new GUIDs.
However, I have created an ETKey Generator for Wolfenstein: Enemy Territory, hosted here at Fearless Assassins.
Okay so step by step I'll explain how to get your new ETKey, and how to use it.
You will be prompted to Open or Save the etkey file. Save the file.
Once it has downloaded (this should only take a second or two as it's a very small file), copy the etkey file to the relevant folder:
WinXP: C:\Program Files\Wolfenstein - Enemy Territory\etmain\
Vista/Win7 64bit: C:\Program Files (x86)\Wolfenstein - Enemy Territory\etmain\
Vista/Win7 32bit: C:\Program Files\Wolfenstein - Enemy Territory\etmain\
Mac OS X: /Users/username/Library/Application Support/Wolfenstein ET/etmain/
Linux (*): /home/username/.etwolf/etmain/
*Based on Ubuntu setup.
Note: On some installations of Windows 7, you may also need to copy your new ETKey file to: C:\Users\username\AppData\Local\PunkBuster\ET\etmain\
You should now be good to go - start up wolfenstein and you should have a new GUID. You can check by typing /cl_guid into your console and seeing if it displays any letters/numbers.
Not working?
If /cl_guid reveals that you have an *Unknown* guid, join a server then type the following:
/pb_cl_enable; pb_system 1; pb_writecfg; reconnect;
This should solve all issues.
Pro tip: to save this problem in the future, read my tutorial on Backing Up & Restoring your ETKey!