So my 17" PB was spitting out every disk I put into it, regardless of brand or type. I called Apple and did some tests and they couldn't think of anything other than a drive failure and decided I should bring it it.
Unwilling to let go of my PB for a week or more, I've been "fiddling" around with it to see if I could fix it. And I have.
How you ask? By squeezing the case at top and bottom over the optical drive. And voila!
I knew the drive was still partly working, but felt there was just some sort of error with how the disk was going in. Turns out my suspicions were correct.
So if you have this problem in the future, and there's no other apparent cause, give the squeeze a shot. But don't be a hero. Use a reasonable amount of force. No need to crush it.