Almost every virtualization admin that I interact with has materially changed their strategy at some point with their first generation of server virtualization before the entire project is complete. Among the strategy changes are those related to network zoning, which will become a more important consideration as organizations approach higher levels of virtualization.
Specifically, the placement of external facing systems on the same virtual host as systems which house internal systems can put both sides of the network at risk if a compromise is made to the hypervisor from the external facing systems. This becomes especially important as the virtual appliance space allows organizations to easily consider firewall, intrusion detection, VPN and other external facing roles to be placed into the virtual environment as well as the frequent goal to virtualize everything.
A more isolating strategy creates a separate environment with hosts dedicated to hosting virtual machines (VMs) that are external facing and not simultaneously host VMs on the internal network. While the hosts may be connected both to the internal and external networks in a DMZ network role, a compromise to the hypervisor or host system would not have as direct of an impact to the VMs running only on the internal networks. This also helps in emergency remediation by allowing a virtual host to be fully isolated or powered off until the issue is identified without impacting the internal network VMs.
When planning your next generation of server-side virtualization, consider the risks of placing internal and external network zones on resources that may contain external facing and internal only VMs. This type of architecture can bake in some inherent security into your environment that may save the day in the event of a zero-day vulnerability situation that affects the guest operating system or the virtualization hypervisor.