I had a problem similar to that, but it was only for one album. I believe the problem occured when I put that album into WMP to get the album art for it (this was before iTunes had the built-in feature). I think WMP did something to the files that didn't allow me to add them to iTunes.
I resolved the problem by replacing that album with the copy I'd put on my brother's computer (we don't share music across the network, instead we just copy the albums across machines after we rip them). Re-ripping the CD probably would've resolved the issue as well, but that would have taken longer.