Rockstar has always been very friendly with Sony and with Playstation, and I don't expect that to change any time soon. They took Microsoft's money for the exclusive DLC, but I'm guessing released it as soon as they could on the PS3. Hopefully it sells well. I also remember comments made after GTA IV's launch Dan and Sam Houser stated that fitting the whole game onto the 360 was challenging, and that they were worried about fitting the follow up onto 360 at all. So, yeah, Rockstar likes Sony.
Now, that being said, I don't expect them to fully concentrate on PS3 at all. They make too much money on 360. GTA IV sold more on 360, and up until MW2, GTA IV was the most successful game launch ever. That was in large part due to 360 sales. So, I don't think we'll see too many Rockstar PS3 exclusives, but I also don't expect them to ever leave Sony out in the cold.