The only reason GTAIV is on the 360 the same time as PS3 is because Microsoft made a deal with them around E3 of last year. It wouldn't make any sense for them to start the project on 360 hardware if they didn't think the 360 version would come out at the same time. In the interview they were talking about the engine and how they could use Cell and Blu-Ray to stream so many pieces of data simultaneously. And Rockstar has stated that they specialize on Sony hardware. Criterion has said the same thing, and they have come right out in saying that PS3 is the lead platform for Burnout 5.
Like Slim said, it might not even matter in the end. But porting from PS3 to 360 and vise-versa is no easy task. Just look how long it took to port Oblivion and get it to actually take advantage of PS3's hardware.