I recently installed a new eMac and during the afternoon, a CD I had put in the tray refused to mount and refused to eject. I tried eveything (and I mean everything) but no joy. A quick call to Apple tech support showed me I hadn't tried everything after all.
Turns out something I had installed had made the eMac 'forget' it had a CD drive, and a simple zap of the PRAM sorted it out.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030128064454402