If you have 10.5 Leopard installed, Disk Utility can add/modify partitions on-the-fly, without erasing the contents of existing partitions.
In Disk Utility, select the device you wish to add a partition to. Then, click the "+" to add a 2nd partition. You can then drag the frame to resize the new partition (or type in the size). After you've done that, click on the original partition and make sure it says "This volume will not be erased". Selecting the new one, it should say "A new volume will be created". With the new partition selected, set the format type. You'll get a 2nd chance to verify all the settings when you click "Apply".
I'm not sure what format Ubuntu will need for the install. I suspect free space, but, you're on your own there (I've installed on "PC"s, but not Mac).
Be sure to carefully read the information displayed in Disk Utility - it explicitly states whether or not a partition will be erased.
Another option, which is a bit more versatile, is something like "iPartition" (Coriolis).