Phil Harrison has been speaking at GDC about the lack of third-party exclusive titles for PS3, saying that they will be easier to secure once PS3's install base grows.
Speaking to various bloggers at the conference, Harrison said, "I don't think PS3 has the install base to support Rockstar's investment in GTA IV on its own." And if you think about it he's pretty much bang on the money.
Grand Theft Auto IV is going to cost Rockstar a lot of money, especially as the company is taking the technology in the game further than it has in the past. And to that end it needs to make a lot of money from sales of the game. Putting it on a platform that's very early on in its lifetime with a small install base won't make you as much as splashing it across two platforms.
Check out first details on the GTA IV trailer here.