Doh. I just fired up iTunes, and my tunes were gone. The tracks are still all on my hard drive, but the playlists and songs in iTunes are missing in action. I've been using iTunes since before it was iTunes (I was a registered owner of SoundJam MP, which was purchased from Casady & Greene by Apple to become iTunes 1.0). This is the first time I've lost data in any of my SoundJam/iTunes libraries, in something like 8 years (roughly 8 major OS updates, over several computers).
Re-added all of my tracks to iTunes (dragging the folders into the iTunes Library), and it sees the music again, but my carefully hand-entered and automatic metadata is gone. My Rating. Gone. Playlists. Gone. Play Count. Gone. Last Played. Gone. My "Pick the top rated songs that have been played a whole bunch in the last month, and give me a playlist that will fill an audio CD with them" playlist. Gone.
Now, assuming I recreate all of my dynamic playlists, which used this metadata to do their magic, I still have to manually re-enter a whole bunch of metadata (especially Ratings) for them to be useful. Crap.
I've never been a big fan of metadata - it needs to be hidden and under-the-hood for it to be effective. But, things sure get messed up when it's yanked out from underneath you.
My copy of iTunes now functions at about the level of WinAmp. It's had a pretty serious lobotomy, and will take some time to relearn what I like etc...