One of the most exciting new features of vSphere 4 is the ability to use a virtual switch from a 3rd party networking vendor, and the first instantiation of this is the Cisco Nexus 1000V.  With this in place, network administrators who are used to working with Cisco devices can now monitor and manage the virtual switches on ESX just like they would any other physical switch.  They can also set security policies and configurations that they know and are used to working with.  This is an important aspect of being able to use vSphere in more security-sensitive applications, such as the DMZ.

Recently, Cisco and VMware jointly released a white paper on virtualizing the DMZ with vSphere 4 and the Nexus 1000V virtual switch.  From the abstract:

This paper tackles the subject of DMZ security and virtualization. It covers a number of DMZ security requirements and scenarios, presenting how vSphere users can implement the Cisco Nexus 1000V virtual switch in a DMZ.

You can download the paper here.  There is also another paper which goes over the general issues of virtualization with mixed trust zones, available at this link.