As it turns out, I too am experimenting with Ubuntu and Yellow Dog Linux on my PowerBook (and have had to reinstall Mac OS X three times, so far, but that's another story) so I know just what you're talking about.
For Ubuntu, which is based on Debian Linux, the bootstrap loader - the program that decides which OS you want to boot - the program that controls that first few seconds of your computer startup is called yaboot and it's actually a project that lives at http://penguinppc.org/bootloaders/yaboot/.
To modify yaboot so that the default operating system is actually Mac OS X isn't too hard. There's a great yaboot reference document online at the yaboot site. It turns out that the change is trivially simple: in the file /etc/yaboot.conf on the Linux side of things, you simply needed to add the line defaultos=macosx.
The second - and critical - step is to actually install the new bootstrap loader configuration file, and that's done with ybin -v which figures out where the new configuration file should be moved and does it.
And keep your eyes open as you go along: the most amusing line in the entire process is the output statement "Blessing /dev/hda6 with Holy Penguin Pee". Only in the world of Linux!
Sign up for my weekly AskDaveTaylor Newsletter and you'll receive even more tech and gadget help
right to your inbox, along with exclusive news and industry updates. It's good stuff. I promise!
i found something interesting dualbooting Ubuntu with a Mac on http://www.ubuntux.org, but I can't recall the exact URL.
but i could solve my problem there.