I usually convert my music to m4a and then create a disk image of the files, mount the disk and drag the image into iTunes' library. Then when I double click a song iTunes quickly and automatically mounts the image and starts playing the song.
The problem is that iTunes 4.1 doesn't mount any new images I place in the library, but it works fine with the old images.
Any ideas? Any ideas on how to get iTunes to unmount images automatically?