They have a statement in their news section from Rockstar saying;
"We know this is about four months later than originally planned and we know that this short delay will come as a disappointment to many of you, but, trust us, it will be worth the extra time. GTAV is a massively ambitious and complex game and it simply needs a little more polish to be of the standard we and, more importantly, you require.
To all Grand Theft Auto fans, please accept our apologies for the delay, and our promise that the entire team here is working very hard to make the game all it can be. We are doing all we can to help ensure it will meet if not exceed your expectations come September - we thank you for your support and patience."