THQ is following EA Sports' lead by requiring a code for those who want to play UFC Undisputed 2010 online. Just yesterday another publisher, Ubisoft, said it was considering doing the same thing.
UFC Undisputed 2010's online functionality, including the new Fight Camp mode (where you can get together with 40 other gamers and train your fighters), requires a code that is free for new versions of the game. Unlike EA, however, it'll only cost you $5 if you have a used copy of the game (where the code has already been redeemed) and want to play online.
This is clearly the way of the future for publishers who want to try and make money off of the used games market. Perhaps all we can hope for is an Online Pass Price War, where companies like THQ try and undercut its competitors by offering a cheaper pass.