This is a great how-to from Scott Lowe’s blog. It walks you through creating a bootable ESXi USB stick on a Mac.
Great for using ESXi on servers with no hard drives in them and great for learning/seeing ESXi in action.
http://blog.scottlowe.org/2009/01/08/creating-a-bootable-esxi-usb-stick-on-mac-os-x/