Don't forget to backup your iTunes Library files before upgrading to 4.9!

When my brother upgraded to iTunes 4.9 on his Mac Mini running Tiger, his library got erased and might have been corrupted. iTunes 4.9 uses a newly named binary file. It's now "iTunes Library," rather than "iTunes 4 Music Library." (Perhaps an indication of what to come and/or a response to 4.8's addition of movie support) The old iTunes binary file is moved to the Previous iTunes Libraries folder. The story is quite complicated, but we were able to get it back for a moment only to loose it forever. Diskwarrior only complicated matters and further screwed up what already was a corrupted file. Our biggest mistake was probably canceling reimportation when iTunes itself recognized the binary file was corrupt. When iTunes wishes to reimport your music spontaneously, let it, because it has a good reason.
But above all, the lesson of the day is to backup that iTunes library, especially before iTunes upgrades. Often people will overlook the iTunes library files when they backup their documents. My brother's last complete system update, which included his iTunes library files, was a good six months ago.
-Nate (not moose)