Something which I've only found out today due to the fact this appears to have had no coverage whatsoever on the internets, not even from Major Nelson, is that the second GTA IV theme is now up on the Xbox Live Marketplace. It costs 100 points and features the artworks you've been seeing recently, the characters on a white background with some part of the city on it, and in my opinion it looks a lot nicer than the first GTA IV theme.
Since there has been nothing about this anywhere, and not even the xbox.com page has been updated, I have taken some photographs of the theme off my television, and they're viewable on my flickr photostream linked below, click the thumbnails for bigger images.
The theme was probably put online sometime this week, perhaps along with the xbox.com exclusive advert. If you want to download the theme you can, by going to the marketplace blade and straight to new arrivals. If you're reading this post and the theme is no longer 'new' you can get to it by going to game store > themes and gamer pictures > all themes.