Well... free for those who have purchased all 5 DLC packs.
 
Got the info from Gametrailers
 

 Today Bethesda and Microsoft revealed that a premium theme for Fallout 3 will be released on Xbox LIVE on Oct.1 free of charge for all players who have purchased all five Fallout 3 Game Add-ons as of Sept. 22. Gamers who have purchased all five Game Add-ons (English only), including “Operation: Anchorage,” “The Pitt,” “Broken Steel,” “Point Lookout” and “Mothership Zeta”, will receive a token from Microsoft to download the theme for free starting Oct. 1. “This is something we’ve been asked about a lot and we’re happy to finally be able to bring it to fans as a small token of our appreciation,” said Todd Howard, game director and executive producer for Fallout 3.The theme is also available for purchase at 240 Microsoft points.

I support bonuses for people who support games.

The problem with premium themes -- and almost every theme -- is the same problem with a lot of people's desktop wall paper. I just want it to be pleasant to look at. It shouldn't be distracting and conflict with all the other items I'm trying to look at. It shouldn't look like a clip-art package puked all over the screen or like I'm trying to decipher content overlayed on a giant comic book background. But they never seem to understand this. Themes seem to just be filled with giant ridiculously busy backgrounds.
 
Of course, a lot of that is due to the limited options Microsoft's interface provides. You can only customize so much and the wallpaper is about the only way to set one pack apart from another.

At least I get something back. 
 
I mean, besides all those hours of entertainment.