OSXFAQ - Technical News and Support for Mac OS X
There are several odd OS X weaknesses. One of them is trouble ejecting CDs or DVDs. In some states, esp. with fast user switching, a CD fails to mount and it cannot be ejected through standard means. The CD is "stuck" in limbo.
This page has a number of tricks. The one that recently worked for me was to fire up Disk Utility. There, and only there, I could see the CD. Then I was able to select it and eject it.