The OpenStack Install and Deploy Manual for Essex seems to imply that the whole stack can be deployed on one machine without the need for VMs. In fact, DevStack does exactly this.
However, the Openstack Basic Install for Folsom says "You need at least three machines (virtual or physical) with Ubuntu 12.04 (LTS) installed."
Is this something that has changed in the architecture? DevStack doesn't seem to have any problems running on a single machine, so why would a "proper" install of OpenStack? As I understand it, all the components are separately-running non-conflicting processes. The only thing that I can see being a problem is the potential for networking conflicts.
Are there things to watch out for by running it on a single machine?