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OK, first off, have you tried just right-clicking on the icon on the desktop and hitting "Eject" ? If that fails, open a console and try typing "umount -lf /media/cdrom" (assuming that your CD drive is in /media/cdrom if it's not, substitute yours in there).
To explain the command above:
The command "umount" - stands for "un-mount". The options "-lf" means "Lazy force". The force option means force it to unmount it, doing (almost - see below) whatever it takes. The "-l" option (very useful for CD's, not good for external drives) stands for "lazy" which means that it will sever the connection with the drive - so if something's reading from it it'll just sever that connection (sometimes the -f fails because the drive is locked by something reading, this gets around it). This isn't good for drives where you're writing, as it will corrupt whatever was being written at the time (it *just* breaks the connection, doesn't wait for it to finish writing).

I hope that wasn't too rambling or complicated (this is what comes of doing 1/2 then coming back 15 minutes later...) let us know if it works or not, or if you need some clarification.

Penguin