How can one create a bootable Linux USB key that works on Mac (Intel 64 bit CPU) hardware?

From Boot Linux from USB on MacBook Pro 17″ :

It works! Can now boot Linux on an unmodified MacBook Pro (5,2) from a single USB stick (or external hard drive).

I created a small 25MB partition as the first partition, formated it as Mac OS X Journaled and installed rEFIt to it, then followed that partition with a 100 MB boot partition, and then a root partition on a thumb drive (if using an external hard drive you could create swap as well, but I don’t do that on thumb drives so they don’t get worn out).

Install linux normally (debootstrap is how I do it) onto the second and third partitions. Make sure Grub is installed to the MBR of the drive and points to the /boot partition as the second partition. Then take the USB drive to a MacBook Pro and insert it.

Turn on the MBP and hold down the Alt/Option key until you see a boot menu offering Mac OS X or rEFIt. Choose rEFIt. After that you’ll be give a menu to choose OS X again or Linux. Here, choose Linux.

Tada, now comes the Grub boot screen and then on into the Linux of your choice. Congrats and enjoy…

Some other useful articles that contain detailed instructions:

• Install Linux to and Boot from a USB Drive on Your MacBook
• How to create an Ubuntu live USB for both Mac and PC
• Installation of Ubuntu Karmic Koala for a MacBook Pro 3,1 on a USB stick