I remember reading somewhere that, Sony probably had a deal with Rockstar that said they couldn't release IV on just the 360 and release at a later time on the ps3 it had to be day and date. And if the PS3 version underperformed they had to delay both versions.
On another note, the preview that got released this week were all from the 360 build and it was not even near perfect so I'm guessing neither version was actually good enough to release last year.
I also wonder when will multiplat titles look better on the ps3, if ever. .