I just saw this quick post that shows you how to identify virtualisation candidates using Operations Manager 2007 and 2007 R2.
OpsMgr is constantly gathering performance information from monitored servers. This information is archived and stored for just over a year if you use the reporting database. The VMM reporting pack allows you to report from this data to identify machines that might be underused as dedicated servers and are more suitable as virtual machines.
Obviously, there’s still some human decision making that’s involved in the final process.
Once you’re happy, you can use VMM 2008/2008 R2 to P2V (physical to virtual convert) your physical server so it becomes a virtual machine on a Hyper-V host. Some OS’s or server deployments mightn’t be suitable so you might look at using something like the cloning solution from Acronis to convert the machine into a VM.