I myself can only spot an error in the developer code thing at the top right corner, which is somewhat cut off on the TV. Otherwise, I can't see the "obvious" fakeness in the shot. All games in developement, especially one like GTA4 which hasn't had any true released screen shots or even artwork to compare, end up looking much different in their finished state than in their development time, so the light cartoony look is sorta explained. The dog and the pedestrians look somewhat retarded, but again, development. All GTA games usually use the same car set, with a few mild changes, so the appearance of older cars isn't exactly a big deal, even moreso if you're near a ghetto area.
While I highly doubt it's real, I wouldn't rightly have dismissed it so early if the entire GTA community wasn't bashing it one at a time for a small detail each.
Also, I could easily label the 3D radar and, with a bit a tweaking, could actually be a neat addition. Just have it that tall buildings "fade out" when they'd get in your way, and make it optional so older fans won't be totally alienated.