Mortal Kombat 5 is official--and it is preferred for GameCube.
Midway recently put out a press release during E3 stating that Mortal Kombat 5 is in the works:
This new version of Mortal Kombat is scheduled to ship for Nintendo GameCube⢠and other next-generation game systems in Fall 2002.
Midway went on to mention:
This title is not yet proposed to Sony Computer Entertainment America.
It looks like Midway really likes the GameCube, and believes it could be the new standard!