Installing Ubuntu on Mac
Resizing the Mac partition
Before you can install Linux, you need to use the Mac to resize the partition.
To repartition your drive, it's important to delete / temporarily move files that are large (like bigger than 1 GB). To help you find them:
Also, it's useful to delete the Mac's "sleepimage" file. 
sudo rm /var/vm/sleepimage
Download the image from http://wiki.ubuntu.com/PowerPCDownloads
Be sure to burn the disk using Apple's Disk Utilities. (Right/Ctrl click the file and select Open with... Disk Utility). Open the image file (from the file menu) -- select it and select Burn...