Microsoft would rather have an awesome GTA4 in April than an unfinished GTA4 in November/December. They still have a good holiday lineup.
Sony, I'm not sure. I'll be honest, I don't know what their holiday lineup looks like, but I know they need every solid title they can get. I doubt, though, that anyone was going to buy a PS3 to play GTA4 in the first place, especially since it has exclusive content on the 360. So I don't think this hurts Sony too much either way.