What is the simplest way to dual-boot linux? I am looking for the most straightforward way (both in initial setup and in maintenance) to set up a second partition on an OS X machine for (either) Debian or Ubuntu. I won't be running any other OSes so the ability to triple boot is of no interest to me.

Most articles suggest installing a third-party bootloader, either Refit or Refind, as one of the first steps. Is this really necessary? In the past, I've just partitioned some empty space with Disk Utility and let the Ubuntu installer format and install to it however it pleases. Is there something wrong with this approach?
Formatting a partition with Disk Utility on OS X and letting the Ubuntu installer do the rest of the work is a simple and straightforward approach.
The Ubuntu installer also takes care of configuring and installing the bootloader.

Theoretically, you could boot Linux with only Grub-EFI, as the Linux kernel supports direct EFI boot. But this requires a lot on work on Macs, because you need to configure and install grub-efi. See also this page.

So i recommend using rEFIt or rEFInd in order to boot Linux on a Mac.