There is a long history of problems with leaving CD's in the drive across a reboot. I don't remember too many issues under os x, but under 9 (and earlier) there were quite a few "problem CD's". If you left these in the drive, the mac just wouldn't boot.

In general, I think it's a good idea not to leave a CD in the drive. It can be hard to get it out if you can't boot (on some models), and if the CD prevents you from booting in the first place...

On my old iMac G3, Apple told me to always leave a CD in the drive, otherwise problems would arise. I remember the CD tray vibrating and whatnot.

Hold down the mouse button during a reboot - it will spit out the disk.

failing that hold down command-option-o-f and at the prompt type "eject cd" press return, remove disk and type "mac-boot" and press return.

that should cover most macs.