Microsoft sent out a press release earlier confirming a couple of things. Firstly, The second GTA IV trailer, "Looking for that special someone", will be available on the Xbox Live Marketplace tomorrow, shortly after it's debut on RockstarGames.com and Yahoo! Xbox 360 owners will of course be able to download this free of charge.
Secondly, Microsoft also added that they would be offering an exclusive GTA IV theme for 360 owners to download, as well as gamer pictures.
To celebrate the release of the second trailer, Xbox 360™ gamers will be able to download exclusive Grand Theft Auto IV theme and gamer pics from Xbox LIVE® Marketplace.
It was not mentioned if these will be free of charge or not, however, based on many other games, it is likely there will be a small charge for them. We'll let you know if we get more info on this.