Same for me. 10.0 on 10.6.4.
Don't forget to +
give Apple feedback+ so they know the details of the situation... what files cause it, which OS X versions it happens with, how widespread this issue is, etc.
Sometimes the little progress window flashes as if it's going to copy the tracks into my iTunes Music folder (particularly if I launch iTunes by opening the tracks), but that window always vanishes instantly. No messages in console... no errors in logs or on stdout.
If I 'drag' the tracks to the song section or the Library in the sidebar, it does outline the section and put a plus on the cursor, but nothing ever happens when I 'drop'. They play fine in QT Player, etc.
I grasped at the straw of the earlier reports that the Library location would have been lost, but my Library has always been in the default location. My location was still showing correctly when I checked it, but I changed it to my Desktop (scary!) and changed it back. Everything already in iTunes still works but the switch and switch back didn't help.
Ah, the pangs of being an early adopter.