What is the simplest way to dual-boot linux?
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.