I know this thread is a bit dated now, but just in case someone looks it up in a search or something:
A lot of the game data such as how much money to start with on various difficulty levels, stats for the weapons, base facilities, aircraft, etc., are in the .ufo files if I remember correctly, and they can be unzipped from the pk3 files and modified to alter the game quite easily with a text editor (such as notepad on Windows).
Therefore, there really isn't any need for release versions to have cheat codes IMO, because if one is playing the game and not testing a feature (as with development), if they find it too difficult or want something to work differently, it is very easy to change it.
The game, after all, is open-source...