Hold Control-Command-Option-Eject Button; this will shut down your computer. Turn the power on with the power button and hold Command-Option-O-F -- this will boot you into open firmware. Now type eject cd and wait until the CD pops out. Type mac-boot and you are ready to rock!
[robg adds: As mentioned elsewhere on the site, you can also try holding the mouse button down during boot to force the system to eject any inserted CDs.]

