We've recently decommissioned our last physical domain controller. We used to run two physical Windows Server 2003-based domain controllers. A couple of years ago, we P2V'd one of them; it's still in production but will be taken down next summer. A couple of months ago, we deployed two new Windows Server 2008 R2-based virtual domain controllers as new installations and took down the last remaining physical DC. It's not recommended to P2V domain controllers, but we've run into no problems in more than two years, so we dodged a bullet.
Have you done something similar? Do you have all of your DC's running virtual?
If so, consider the following articles from Microsoft and VMware, respectively:
All of the DCs in our environment are virtual guests. We did not P2V, but instead built new VMs for the DCs, promoted them, and then migrated the FSMO roles before decomissioning the physical servers. We have 2 forest root, 2 child domain DCs, and one RODC in the DMZ.
We followed a similar process at several customer sites, creating new virtual DCs to replace the existing ones rather than perform a P2V. When we migrated the DNS servers, we first created an extra, but not "public" DNS server on a DC (just as a safety measure), shut down one of the DC's running DNS, then promoted & configured the replacement, using the same IP address. We used this procedure to virtualize all but one of the 8 DCs at a major US travel company, including DCs in the US and Australia.
Personally, I feel that the ability to snapshot DCs prior to upgrades, and use image backup through VM provide significant reasons to justify a virtualized AD environment.