Right, Up, Left, Right, Down, Left, Up, Right, Down, Left, X, Y, Y, X, Start...
If you pressed in that combination on the main menu screen of UFC Undisputed 2010, you got the playable character of Shaquille O'Neal in the Heavyweight Division. Shaq is known to be a huge UFC fan and had the physique and popularity to be in the game as a secret character.
UFC 3 will be coming out in three-and-a-half months, which should be enough time to add in a downloadable or in-game secret character. Here is a list of 10 potential athletes/actors who may make the cut.
(Sidenote: UFC 3 comes out on Valentines Day, I suggest getting a girlfriend soon and dropping hints at her pre-ordering it for you.)