Problem getting Mac mini back from ubuntu

Well, I decided to try ubuntu 7.10 out on my Mac mini, so I downloaded 
and burned the 7.10 live CD.  figured that way I could test it live 
without actually using up a drive or re-partitioning to install it.

It seemed easy at the time, I set the startup disk target in System 
Preferences to be the 'foreign' OS on the CD drive and restarted. Ubuntu 
started up and I took it for a test drive.

Then everything went sour. I quit Ubuntu by choosing restart (thinking I 
could remove the CD) and restart from the hard drive, back into OSX.

Wrong .... now it seems to be constantly trying to reboot into Ubuntu. 
If the CD isn't there, it winds up with a large fla****ng underscore 
cursor in the top left and won't respond to anything.

I've tried:
rebooting into Ubuntu and shutting down (as opposed to restart)
restart with option key pressed
restart with command-option-p-r
restart with ****ft key pressed
restart with command-s

Nothing works any more, almost as if it's been hijacked by ubuntu which 
is not allowing any other OS choice. All of the above result in the 
black screen with the fla****ng cursor.

What'd I miss ?

Anybody know how to get rid of Ubuntu so I can get back to work ?