You could also just use the Disk Utility from your OSX CD/DVD... much easier than the text-based tools provided with the Debian base installer.
*Boot up from the OSX disc, launch Disk Utility.
*Use the graphical slider to create 1 partition for OSX: Mac OS Extended (journaled); (i believe that's the default format)
*On the 2nd partition, opposite the OSX side, make sure that it indicates that it's unused (ie. no format listed for this partition).
It's up to you whether or not you want:
*to press and hold Option while booting to select either OSX or Linux immediately after the Mac's startup chime, or...
*to use the Yaboot startup prompt to select either OSX or Linux
If that's the case (you want to hold down Option @ startup):
*Install OSX on the 1st partition, right after using Disk Utility;
*Install Linux on the 2nd partition;
If you want to use Yaboot to provide you the startup menu:
*Install OSX on the 2nd partition, right after using Disk Utility;
*Install Linux on the 1st partition; all you have to do is boot into your Linux CD, then when you get to where you want to install Linux, just specify that you want to use the largest unused disk space; this should install Linux on the empty partition.
*At the end of the Linux installation, and after you reboot, you should be presented with a text prompt that lets you select either Linux or OSX (there's a time limit for you to select which one you want), otherwise, it'll default to Linux, i believe...... you can edit it to boot into OSX by default if you wish.... the instructions are on the forums somewhere....
After you've gained more experience, you can probably create 3 partitions (1 OSX, 1 unused for Linux install, and 1 FAT-32)..... the FAT-32 could be used to share/copy/manage files between OSX and Linux...
Need to start in "Single User Mode"?
Listening to: http://www.youtube.com/watch?v=5EaRHlo4B0Y