Just a small update to the previous announcement that GTA IV will be sold via Steam, an online game distribution service. It is now possible to pre-purchase GTA IV and if you do so, you will get Grand Theft Auto: Vice City for free! If you already own Vice City on Steam, you will be able to give your free copy to a friend, possibly as a way to get them into the series.
The game hasn't been available for pre-purchase very long and it's already high on the top-sellers list. (Click 'Top Sellers')