Also interested. I don't have an AppleTV yet, but considering one. For slideshows I'd like to play music that I was listening to at the time (eg. for our '04 vacation photos, I don't want the '08 tunes I just downloaded).
Addition: I just tried this with Front Row and it works (using Front Row) -- create a slideshow in iPhoto, select the music playlist and save. I made a second slideshow with another set of music. Both slideshows were saved with separate music playlists and played back as such. I didn't have to change any settings once saved.
Does AppleTV function differently?