I am no expert on Macs but if you down load the
Ubuntu Pocket Guide there are instructions on page 17 on how to install Ubuntu with Boot Camp.
Apparently you create a 5GB (or bigger) partition then when Boot Camp Assistant finishes shrinking the OS X partition, hit the QUIT & INSTALL LATER button.
Delete the NTFS (Windows) partition created by Boot Camp Assistant.
Download and install the rEFIt software. This provides a boot menu that lets you choose between Mac OS X and Ubuntu. rEFIt can be downloaded from
http://refit.sourceforge.net— choose the Mac disk image download. NB There is no immediate sign that rEFIt is installed.
Insert the Ubuntu install CD and reboot your computer. when you hear the boot chime, hold down the C key. Eventually, you’ll see the Ubuntu install disc boot menu. Follow the instructions on screen, but when the partitioning stage begins, don’t resize. Instead, select Guided- Use the Largest Continuous Free Space. (There is a bug with Ubuntu 8.10 when selecting the Guided‐Use the Largest Continuous Free Space option shows Ubuntu occupying the entire disk in the “After” graphical preview. This wrong and can be ignored.)
Follow through the installation stages, as if installing on a normal computer. When you reboot, select the Linux penguin on the rEFIt boot menu to boot Ubuntu, or the Apple icon to boot OS X (use the cursor keys to highlight either, and hit Enter to select).
Hope that helps.