i later copied a bunch of tracks to the ibook and they all worked without additional authorization. so its not something on a file by file basis is my guess, it authorizes the computer, not the track. my guess is that the file gets your data jammed into it as it downloads, which ties it to you.

also, in QT Pro, Export is disabled for these tracks, so if you Save As... you notice that the soundtrack is still protected, you're not recompressing the file. the first bummer i've noticed is that even with the new QT, FCP 3 won't import the AAC files.