Basically when you want to test ESXi 4 installation to the USB Stick you'll want to follow my post, which I've done in the past and that's it.
But if you want to test unattended installation of ESXi on USB stick then you would want to boot your VM which is executed in your VMware Workstation, from an USB stick too.Well you can't. Even the version 7 of VMware Workstation does not support that.
Here comes the help tip from Ivo. Basically what do you have to do is a work arround.
01. Download boot manager from PLoP
02. Use the Iso in the boot manager as a boot device to boot your VM
03. Add an USB adapter to your VM (if not already done).
03. Insert your USB stick with ESXi installed in it.
03. When your VM boots from that ISO, just select USB from the menu.
Well not all the credit goes to IVO, because the original help provided was from Ulli Hankeln from sanbarrow.com. -:)