A bizarre problem recently popped up after I had updated to iTunes 7.2. Suddenly, certain songs would no longer sync with the iPod, even though I had ripped them from my own CDs. These weren't iTMS purchases. And there were only 4 mp3s out of a library of nearly 4K songs that wouldn't sync over anymore.
The solution was simple (but stupid on Apple's part):
In iTunes, drag the songs to the desktop.
Then delete them from iTunes.
Re-add the files back in to iTunes.
No clue why this happened.