There's another topic about this, but it has a misleading title about the AO rating.
Here's the deal: there are several hump-your-girlfriend mini-games in the San Andreas program code. You don't see genitalia in this sequences, but you see everything else.
The mini-games were disabled before the game shipped and replaced with innocuous cut-scenes, but a clever hacker has created a patch to reactivate the mini-games in the PC version. People are freaking the hell out.
The ESRB is investigating the matter, as confirmed by Rockstar, which didn't actually 'fess up to the existence of the mini-games: "We feel confident that the investigation will uphold the original rating of the game, as the work of the mod community is beyond the scope of either publishers or the ESRB."
An MSNBC article: http://www.msnbc.msn.com/id/8515870/
Screenshots of the mini-games: http://www.gtagarage.com/mods/screens.php?id=28
Discuss, and by discuss, I mean laugh your ass off.
-- Z.