Check out the following guide @ vminfo.nl :
Create an ESX 3i USB boot key
This guide will describe the basic steps to create your own VMware ESX 3i USB key.
I need to say that VMware is not supporting this way of running ESX, so be careful with this on
When you want to use ESX 3i in production environments without the need of local hard drives,
contact your hardware manufacturer; like HP, Dell or IBM. They have supported ways to run ESX
on USB or SD cards.
• USB key of 1GB
• IZArc – http://www.izarc.org/download.html
• WinImage – http://www.winimage.com/download.htm
• VMware ESX 3i (ISO) – http://www.vmware.com/download/vi/