rEFIt is the best way to allow dual-booting on a Mac. I use it to boot Ubuntu from a partition on my external drive, and it works perfectly. Be sure to back up your hard drive before proceeding.
Install rEFIt and make sure it is working (you should get a boot
chooser on startup) Use Bootcamp or Disk Utility to create a partition
at the end of the disc. Don’t worry about what format, just make it
the size you want for Ubuntu and your swap partition. If you you are
using OS X Lion which comes with Bootcamp 4.0 you will need to use the
Disk Utility. Boot Camp 4.0 will only allow you to resize if you
provide a Windows 7 installation disk. Boot the Ubuntu desktop CD, and
select "Try Ubuntu. Start the partition editor (gparted) under System
-> Adminstration. Use gparted to delete the partition you just made in OS X. It should be the last partition on the disc and follows after
the HFS+ partition (OS X). Deleting it will leave an area called
unallocated. This is free space, and exactly what you want. Use
gparted to manually partition the space on your drive for Ubuntu.
First create a SWAP area of about 1GB. Then the rest of the
unallocated area format as EXT4. Apply changes and take note the label
of the EXT4 partition such as /dev/sda3.
Start the Ubuntu Installer from the desktop icon. When prompted,
choose to manually partition. Select the EXT4 partition and click
change. Select to use the space as EXT4 and root (/) as the mount
point. You will also want to check the box to format the partition.
Reboot when done with the install, and in the rEFIt menu, choose the
partition tool. It will attempt to sync the partition tables on your
disk. Then SHUTDOWN the computer (not reboot), and start it again. You
should be able to boot to Ubuntu now. If it seems to freeze on the tux
logo, completely shutdown again and try one more time.
This is a summary of a process described by the detailed how-to process here: https://help.ubuntu.com/community/MactelSupportTeam/AppleIntelInstallation#Detailed_How-To