I thought Rockstar made a deal with Sony that it would be released the same time.
They did, it was done in exchange for the next Rockstar franchise to be exclusive on the PS3. It went something like as Rockstar and Sony have very close ties, Rockstar wanted to release GTA4 on both platforms at the same time and to make this a bit sweeter for Sony they said if they agreed then the next Rockstar Franchise will be PS3 exclusive. Most of this is mentioned in an interview with Jack Tretton about the PS3 rockstar exclusive but i cant find the link at the moment, but like any corporate thing best taken with a pinch of salt.