As I said I think AppleTV connects to iTunes to view TV programs, films converts to AppleTV format... to listen to the music but only one librarie. That system works through shared libraries like it does between mac. So, I guest AppleTV can connect to differents libraries if there are more than one mac (itunes) turn on. This will appear in menu screen.
I was thinking to buy AppleTV but It wasn't sure. I think all the movies must be included into iTunes in the correct format, then it's not for me. I prefer a to keep Macmini always turn on to be able with beoport. For the rest of computer I use Time Capsule, I don't use shared libraries because I have a only librarie on Time Capsule for all computers.
When I share libraries I have to turn on Itunes on every mac, but I can play differents songs on every mac. Then I think that AppleTV will do the same.
iPhone controls iTunes, so you can control differents iTunes, and now AppleTV. If you have an airport express connected to a iTunes, your iphone will control this iTunes and the sound out of Airport Express. But the Aiport Express can be linked to differents iTunes, ok?
I have an aiport express connected to my BV3-32 but with optical cable (converted to coaxial). Only I use it to play DTS songs. iTunes can play DTS albums and it is fantastic. I use normally Beoport with my BC2, but when I want to listen DTS songs I press V.Aux o AV-V.Aux.
I hope I could have helped.