A member of Britxbox.co.uk claims that the delay is due to development problems around the PS3 version and that the PS3 game is currently "unplayable".
"My mate is doing some work on the GTA and the game is running fine on 360 and is finished, they just can't release it becuase the ps3 version is unplayable."
Though this could easily just be another "My cousin's best-friend's sister's girlfriend's dad's nephew heard" type of rumor. His 4982 posts in the BXB community does give him some credibility. However untill we hear something directly from the devs this is still very much a rumor.