Source: videogamesblogger.com

It was revealed at E3 2008 that downloadable content (DLC) for Fallout 3 is exclusive to the Xbox 360 and PC versions of the game, and will not appear in the PS3 version. However Fallout 3 developer Bethesda has remained quite on details as to how the exclusivity came about.
However Analyst Colin Sebastian with Lazard Capital Markets told Edge Online that Microsoft and Bethesda likely came to some sort of financial agreement to leave PS3 DLC out of the equation.
“…I think it’s a very good possibility that Microsoft and Bethesda were partners in this decision. Obviously Microsoft paid up to secure exclusive online content for Grand Theft Auto IV, and online is a cornerstone for Microsoft’s digital media strategy.”
Officially Bethesda had this to say in an e-mail to Edge Online, “We aren’t going to get into the details of the hows and whys. DLC will be exclusive for PC and 360. We’re not going to give any other qualifiers or clarifications as it relates to other platforms.”
The DLC will reportedly be several hours long and will include an entirely new quest line!
“We want stuff that’s going to be several hours. Not just like a one-off thing, but something like where you can download it and play it for X number of hours,” Hines told ShackNews. “It’ll be similar to what we did with Knights of the Nine in Oblivion, where it’s like whole new quest lines, new stuff, that kind of thing. We want to do stuff like that, where it’s adding hours of tangible stuff to the experience. And it plugs into your existing game, so whether you’re starting a new game, or you’re playing for 40 hours, you can go off and play this,” said Hines.
Work on the DLC for the game is well under way at this point, “We stopped doing content a pretty good way before we finished the game in terms of adding new stuff. So we already have folks starting to look into what [the DLC] might be,” added Hines.