I am having the same problem. I'm using 10.5 64bit. Recently, a lot of my files won't go in to iTunes. This includes MP3s that have been legitimately downloaded from MP3 dealers other than iTunes, and it includes MP3s of my own music which I personally encoded with the LAME engine.
Is this some kind of move to annoy people who don't buy MP3s from iTunes? I tried the MP3 validator mentioned above but it made no difference. Whether I drag the files in, or add them through the file menu, they never appear in iTunes.
I'm trying to release my music here! I can't go around giving people MP3s if they won't load in to iTunes.