Can you explain more about what you did to "erase every ID3 tag"?
What you describe is how iTunes works. When you are browsing your library through iTunes, it displays the song information (Artists, Title, Genre, etc.) from it's internal database (which is backed-up up to the XML file). This allows it to display that information without constantly re-reading the actual data from the files. It also makes searching much faster.
When you play a file or do "Get Info" on a file, it reads the ID3 tags from the actual file.
When you edit the song information though iTunes, it updates the ID3 tags stored in the files, so under normal circumstance this poses no problems.
I can't speak for the OP, but I'm almost positive iTunes grabs some info on it's own.
I had a recurring problem with iTunes and I abandoned its use for music in my network partly because of this problem. (the bigger problem is that it's a bit Fisher-Price if you want control over what you're doing, like many things Apple - but that's beside the point)
It goes like this:
All of my music is stored on a Windows 2003 server box. I rip it in FLAC using j.River Media Center, and that gets stored into the FLAC library. There's a custom-developed piece of middleware sitting on the 2003 server which looks at the FLAC library and automatically trancodes any new songs into a shadow MP3 library. It also looks at changes in the MP3 library in terms of deletes and tag changes, and syncs those to the FLAC library. I do a lot of pruning / organisation / changes on my laptops when I'm listening somewhere and what this allows me to do is to take the shadow MP3 music folder offline on a laptop, make changes, and have those changes synced back to the server when I'm back - and
those changes automatically propagate across to the FLAC library. Similarly if I should make tag changes on my main listening PC in the living room which uses the FLAC library, that propagates across to the MP3 library.
So I was using this with j.River Media Center on Windows machines, but when I switched to the Mac I started putting iTunes into the mix, with all of my syncing machines (Win/Mac) running iTunes. It was quite inconvenient as I had to re-import the library into iTunes whenever I added new tracks, but that wasn't all. I'm thankful for Shadow Copy & regular backups because I've had to use both quite often where iTunes has shuffled away ID3 tag info and those changes have propagated to the FLAC library. I'd find many FLAC songs with almost blank tags, which were properly tagged before. And in iTunes it's still showing up fine. I don't know when it happened or in what sequence it happened - but what's clear is that it happened.
Eventually I took the step of removing iTunes from all of my Windows machines, relying solely on the vastly superior (but Touch/iPhone-reluctant) j.River Media Center, and have music sitting on Windows and Video / Podcasts in Mac, and I use separate iPods for either. In addition, the iPods I have which reliably syncs with only iTunes - that's the iPhones and the Touches - now are synced only with video and podcasts on the Mac, which means that if I want to listen to music I always have to carry another iPod. But this compromise works for me.
