Some movies are mixed with a discreet rear center channel and some are matrixed. It just depends. I believe all three of the prequels were mixed in Dolby digital EX which is matrixed from the surround channels.
A-man -- You can't just load an avi into vegas and expect seperate audio channels. You have to demux the vob and then split the ac3 file into six seperate waves. It's really pretty simple and full proof. I've never had it fail.