Is it possible to boot OS X 10.6 from a USB drive with VMWare Fusion 3.1.3?
You need 10.6 Server. It does not help to change the settings to 10.6 Server, you have to use 10.6 Server as operating system on the USB-Stick… or you can try some hacks, e.g. Hackintosh (search for it in Google) in a virtual environment but that's a little bit more complicated you can check

http://wiki.osx86project.org/wiki/index.php/Installation_Guides#VMware_Install_Guides

for this.