Technically what you're asking is doable however if the Host CPU is different enough between the Physical Machines you're going to pass the USB Drive around to it may cause a WPA (Windows Product Activation) issue.
Additionally you'll be prompted each time the Virtual Machine is started as to whether or not you moved it or copied it and you need to reply moved it to avoid other flags that will be set that can help tip the scale to cause WPA issues.
You can add the following parameter to the .vmx configuration file to avoid the question and have it as though you said you moved it.
uuid.action = "keep"
VMware Fusion 4 now dynamically loads/unloads the various program segments and kernel extensions that in pre version 4 where alway running however make sure you set the Applications Menu to either "Never" or "Only when Fusion is running" not "Always" if the VMware Fusion.app is going to be on the USB Drive with the Virtual Machine.
VMware Fusion licensing has changed to allow more the on install however I do not know if this is applicable in a commercial environment so check it out and if it allows it you can then install it on each Host and just move the Virtual Machine around.