Quite a lot of ignorant posts
The cheat protection has nothing to do with the cd checking, you can download a nocd crack and play without the cd.
The cd key printed on your manual generates a unique id number (UID) which corresponds to the relativaly small number of UID's in the auth server's database (around 10 million probably). If it's there and hasn't been banned, you're allowed to play. If it's not there, you can't play. If the auth server is down, anyone can play, which is better than nobody.
A key generator uses the same algorithm to create a key as UT2004 does to check one, and the game doesn't care wether it's on the auth server or not. People can download a key generator and play UT2004 offline or on unlisted servers that have auth disabled.
The amount of combinations that the algorithm can create is in the billions, and the chances of your generated key being the same as a key that was sold are slimmer than winning the jackpot on the lottery. It would take you months to generate a key that would work online if you were trying and checking 24/7.
'Hacking' the key checking process would involve hacking the auth server, something that's never been done and released to the public in any popular online game that uses an auth system.
To sum it up, the only way a cheater that got banned can play the game again is to go out and buy the game again. I'm sure there's a few people lame enough to do this more than once, but I can't see many people spending hundreds just so they can cheat online; and that's fine by me