o_O
Apparently this is one of those games that's going to use some of the new physics engines around, namely the Euphoria and DMM (digital molecular matter) engine. I found a video on YT that explains them, but it explains it for Star Wars: The Force Unleashed, which will use these engines.
YouTube - Star Wars: The Force Unleashed - Unleashing The Tech - try to ignore the SW stuff, and listen to what they have to say on the physics engines themselves. Maybe this is how they will help create their "dream GTA". Should be pretty cool stuff!