With Hyper-V becoming widely used the question in a lot of designs is weather to virtualize your AD infrastructure as well. The answer isn't always as easy and clear as it looks.
First let's see what Microsoft has to say about virtualizing your ADDS Servers. In article KB888794 you can read that it is indeed possible to run your ADDS server in a virtual environment when you follow the guidelines about pausing, snapshotting, backupping and performance, you should have no problems at all.
Now let's see what would happen if you're using a Hyper-V cluster and because you want to take advantage of your High Availabily features of your Hyper-V Cluster, all of your ADDS Servers are running virtualized. As long as your cluster is up and running everything is looking good. But what is for whatever reason your cluster needs to be restarted after a shutdown? You have a problem. The cluster service isn't starting, telling you it cannot contact a domain controller. So you have a cluster that isn't starting because there is no ADDS server available and you have a ADDS Server that isn't starting because the cluster it is running on cannot be started. This is a major problem. To fix this problem take a while, you have to build a standalone Hyper-V machine and manually copy the VHD of the Cluster-LUN. Now you can start the ADDS server and start your cluster.
So when using a Hyper-V cluster it is really important to keep at least one of your ADDS server running on a physical machine or if you want on a standalone Hyper-V server.