I was trying to install x86_64 version of ubuntu 9.04 server on my mini mac. However, mac mini can't handle x86_64 and needs i686. But now I can't eject the cd and I am stuck. After some googling I found out that you need to keep the option key pressed while booting up. This brought up a menu allowing me to boot off the hard-disk. Phew!
I found this today (07/20/09) - while booting up press the mouse button to eject the media from your CDROM drive