Is domain controller virtualization a good thing?
Well, there are pros and cons in putting your domain controller in a virtualize environment.
For the pros:
1. You will have better use of hardware resources
2. There will be better Disaster Recovery for you DC
For the cons:
1. Now you will need to support extra software
2. Another layer of failure is now added
Personally, I think having the DC in a physical environment is a better solution. Because, now a day hardwares are cheap. But, if you are thinking of putting your DC in a virtual environment. It will works, but you will now rely on another software. So, one solution is to put your main domain controller in a physical box AND have the rest DC virtualized.
You might want to think also using the Windows Server 2008 Server Core installation. This will minimize hardware resource for the DC.