In a recent reply on Bethesda Softwork's forums, Jason Bergman explained that the music in Fallout: New Vegas will include compositions from Inon Zur, as well as including parts of Mark Morgan's Fallout 1 and 2 soundtrack. Here's the quote:
All of the new music in Fallout: New Vegas is by Inon Zur, however we do use some of Mark Morgan's tracks from Fallout 1 & 2. They're mixed in with the Inon music in places (so are sound effects from Fallout 1 & 2) and the actual tracks themselves will play in specific locations. We use some of Inon's music from Fallout 3 as well. There's a ton of music in this game.”
So, apparently the New Vegas music mixes in elements from all 3 previous Fallout games. It'll be interesting to see what they cook up, as this is another way they can include things from the past Fallout games and incorporate them into New Vegas.