Booting Mac OS 10.6.7 from an external hard drive if internal drive running Ubuntu?
The myths page from the rEFit author has a lot of information about the subject of booting operating systems that are usually booted via BIOS (i.e. Windows and Linux). 

In particular:


  Myth: You need rEFIt to install/boot Linux
  
  You don’t. The firmware’s built-in boot volume chooser (hold Option to activate it) will recognize Linux boot CDs as well as bootable hard disks and let you boot them. (They may be labeled “Windows”, though.) For triple-booting you’ll get only one item in the built-in chooser, but you can use GRUB, LILO, or NTLDR to act as a second-level menu to choose between Windows and Linux. The gptsync tool can be compiled to run on Linux.


While you can install some Linux distributions on a GPT-partitioned disk, it's much easier to repartition the internal disk and put an MBR partition table on it, depending on the version of Ubuntu you are going to use.

This is all described at the Ubuntu MactelSupportTeam site. The fact that you are going to boot Mac OS X every now and then does not change the approach; in fact it is even recommended on that page.

Before you make any changes, just make sure you have at least one, better two backups of all your data.