When to use a Virtual Firewall

The 2/9 Virtualization Security Podcast was a discussion on when would one use a virtual firewall. This was in response to being told that there are some people that would never use a virtual firewall for anything, and that got me thinking. Outside of the politics involved with using virtual vs physical firewalls, when would you use one? What are the cut offs, and best practices around using virtual firewalls. We were joined by Rob Randell of VMware to discuss this point.