What they are saying is that if you use a certain key to activate Server 2008 virtually then it will not take a s**t on you if you change hardware and require a reactivation. Since VMs move from VM to VM in large virtualized enterprise environment, when you use the virtual key it treats the activation process differently being more lenient on hardware changes in a virtual environment. If you are using a KMS key, or OEM Activation on your virtual boxes then you don't need to worry about it. If you use the key pair method described here (which must be new because I never had any virtual keys), then you use the physical key to activate the physical box and the virtual keys to activate the virtual boxes.