At this point, we are really just allocating the space you want for Ubuntu. The actual Linux partitions will be created later during the installation.
For pre-Leopard OS X, there are no such tools. BootCamp does not run, and DiskUtility will only allow you to create more partitions if you wipe out all the current partitions. However, the underlying commandline utility still exists.
Here's a usage example. Let’s say you want to resize your OS X partition to 200GB and leave the rest of the disk free (for Ubuntu of course). You would open a terminal and type the following, followed by the "Return" key.
sudo diskutil resizeVolume disk0s2 200G
You can read more on diskutil by typing 'diskutil help' in your OS X terminal.