I was fortunate enough to get the heads up from a source I can not divulge, about what kind of game play is shown in the second and third GTA IV trailers. Unfortunately, I have not been able to convince him to let me take a look at, or even upload these trailers because of fear of a lawsuit.
However, what he told me is extremely exciting. In the new GTA, there is a much more lifelike game play element. Cops no longer, automatically know where you are if they are within a certain range of you. In one of the trailers, it shows the main character running into an alleyway away from police and then ducking down behind a car. You watch as the police pass by you. It is also possible in this iteration to literally crawl underneath a parked car and hide. In the trailer it shows your man leaning out from underneath a car with a rocket launcher and shooting a helicopter out of the sky.
What this means is very significant. It means that their has to be a strong AI component to the police. Escaping them will become much more cat and mouse, than the one man army, finding a spray paint shop and the chase is off shtick of the previous games. It also means we can look forward to a lot more interaction with the environment.