Adding MP3s to iTunes 10 doesn't work for some files

In many cases, improper tagging causes the files to go weird places. I suggest you create a smart playlist that shows you “new added” stuff:

I usually have the same problem with mp3 with dubious tagging.

Upgrading for me made iTunes forget where the music library was (I use an external HD)... putting back the directory fixed it for me.... goto advanced settings, iTunes Media folder location, change... select media folder "iTunes Library". If you don't know where your songs are exactly then right-click on a song and "show in finder", that will give you an idea.

I was able to add mp3's etc to itunes after that... :)

It seems like the issue was related to improperly encoded files. I'm not sure if it was the container layout that was malformed or if the audio was incorrectly encoded, or even if it was just in a format that iTunes could not understand, but re-encoding the files as MP3 allowed them to be read by iTunes.