GTA4 is not entirely exclusive to 360, but it does have EXCLUSIVE content for it by RockStar via Xbox Live.
12:22: "Major announcement about one of greatest franchsies of all time." Says its so exciting that he has to take his shirt off. Shows Halo 2 tatto--GRAND THEFT AUTO IV tatoo on other arm. Grand Theft Auto IV October 16 to Xbox 360 ONLY.
12:25: "Microsoft and Rockstar have entered into an exclusive deal to privide episodic content via Xbox Live Marketplace."
12:26: Grand Theft Auto IV will be updated with regular fresh content, more details later. Crowd is stunned, gasps all around.
Sweet, more stuff to elong the game. Maybe this is an indication that GTA4 will be playable online too.