If I have 3 spaces desktops, and a couple full-screen VMs, then I have 5 desktops (assuming I don't have any other full-screen apps open - safari, etc).
The point is to be able to access them using keyboard shortcuts. Pre-Lion, full screen apps lived on their desktop. So you could use Control-1/2/3/4/... to access a full-screen VM. e.g. Control-3 was my Linux VM, Control-4 was my Windows VM no matter which screen I was on. Control-arrow is not a substitute since the number of arrow presses will depend on the screen *and* the number of open full-screen apps, etc.
In my case, this is not a multi-monitor issue. This is a matter of being able to jump in and out of VMs. And I can adapt to use a different key, if necessary. But I want it to be one key and stateless.