Yeah, it's looking almost certain that Sony's big, upcoming PS3 announcement will indeed be a new God of War title. Especially since there's already an Amazon listing for the thing. And a trailer.
A product page on Amazon, complete with box art and trailer (above), reveals the game is called God of War: Ascension. Interestingly, it doesn't seem to use the letters "IV" anywhere, and going by the narration in the short trailer it appears this is going to be a prequel.
The short trailer, which contains no gameplay footage, shows a stylised Kratos struggling against what look like his inner demons. Or hey, maybe outer demons! It also mentions that, before he was a God capable of killing everything, he used to be a man. Who fought not for rage.