whatever you do, don't import 24 bit files to iTunes and convert directly to MP3/AAC or the like...
oddly enough, i did an experiment about a week ago to find out how iTunes handles this... in short, it doesn't. (if you want to hear it, let me know) i had been doing it to make myself refs occasionally, and it seemed like a time-saver.
BUT, to my ears, it sounds like iTunes was truncating beyond the necessary 16bits. i don't know much about perceptual coding though... so maybe that's part of the nature of it...
anyway, i'm sure there are codecs (LAME, i see..) that can handle 24 bits appropriately, but iTunes CHEWS IT UP!
DITHER!