Does there happen to be a problem with the trackpad's button? I only ask because if it is somehow stuck to be held down (hardware problem), the Mac would get the command to eject the optical medium. (I.e.: If you ever want to eject a CD or DVD at boot-time, you hold down the left mouse-button or trackpad button.)
What happens if you select the CD/DVD as the startup volume in the Startup Volume preference panel?