i think this is how you do it, been over a year since i did it myself... at least it should put you on the right track.
1. boot into linux
2. login/su/sudo to root
3. use pdisk on your harddisk, to find out what partition number your OSX partiton has(unless you already know it)
im not sure i remember exactly how, but i think this is the way:
- run "pdisk /dev/hda" assuming OSX is installed on /dev/hda (if you have OSX installed on the stock ATA disk, this is correct)
- then type "p" and press ENTER (p stands for print, i think)
- a list of partitons will show up, find the one OSX is installed on, and remember the partition number.
4. edit /etc/yaboot.conf with your favorite text editor, type in the correct OSX partition. (unfortunately i cant remember exactly what the contents of the file is, but once you open the file, you will understand what to change, hopefully)
5. then run "ybin -v" to update the bootstrap partition with the new settings.
6. reboot