With Fallout 3, the DLC was Microsoft exclusive for a period of time. Bethesda never commented on how long it would be, or why it was exclusive. I'm sure Microsoft paid for an exclusive period. Eventually all the DLC made it to all the platforms.
I believe the Oblivion DLC came out as Microsoft exclusives for a while as well.
I expect the trend to continue. What I don't expect is for me to ever spend money on a Bethesda product again. They continue to release these massive games, but their QA and writing is sub-standard (the writing in New Vegas really stands out in comparison to FO3). I own a PC and a PS3. I don't own a 360 and have no intention of buying one. I don't care to be treated as a second-class citizen repeatedly.