While vehicles in the first trailer for GTA IV suggests a time period of around ~2005, PatrickW on our forums pointed out something very interesting.

One of the notable landmarks that can be seen in the trailer is the Hearst Tower, which in real life is located at West 57th Street on Eighth Avenue to the south of Central Park. As PatrickW pointed out, this tower was finished in early 2006. Could this be a more clear indication that the game is set around 2006 or beyond? We'll wait for the upcoming preview from Game Informer for more information and bring it to you as soon as possible.

Hearst Tower in GTA IV

Related: Wiki, Old Building, Discussion