Ever since GTA 3, the series was released on Playstaion exclusively, and then Xbox Later. Rockstar and Sony had an exclusive relationship. Playstation would get the GTA games when they release, then there would be an Xbox release.
At The GDC 07 Sony Corp. Exec. Phil Harrison let it be known that the PS3 will not have exclusive titles for some time (this includes Grand Theft Auto IV). That's due to the amount of PS3s and games that people are buying.
It means that when GTA IV releases, the Playstation 3 alongside of Xbox360 will have it available for their system.
What better way to make more money by not being exclusively PS3 (they cant really guarantee the quota that Rockstar needs) but being on Playstation and Xbox, they can get numbers.
So you can be on the lookout for GTA IV (idk the release, but the trailer will debut on March 29th)