With Hyper-V we have always recommended that the parent partition not be connected to external networks – as a security best practice. However, this was relatively difficult to setup (especially for core installations). With Windows Server 2008 R2 we have updated the Hyper-V management UI to make this much easier to do, and the default behavior. Now when you create an external virtual network switch you will see the following UI:
Where you need to check the "Allow management operating system to share this network adapter" option in order to get the behavior that we had with the original version of Hyper-V.