In this months Game Informer magazine it says:
Enjoy our Halo 2 cover story, and be sure to check back with us next month when we bring you perhaps our biggest cover story yet. What game is it? I can't reveal this monster title just yet, but I can promise you that it's been something you've been dying to see, and we'll have the exclusive first look.
Now this could well be San Andreas, remeber Take-Two said the first previews would be in June. And also Game Informer had the world exclusive preview for Vice City so theres no reason to say they wont have the world exclusive for San Andreas.
I don't recall hearing about any other 'monster titles' that are being released soon so I cant actually think what else it would be
Lets just hope it is
It could quite possibly be GTA:SA. It might have been GT4 but they have already released a good many screenshots and information about it.
Good chance it will be SA. I can't think of anyother big game coming out soon that hasn't already been explained in detail (Metal Gear Solid: Snake Eater, GT4, etc.).