Based on your requirements,
Based on your requirements, you will probably want to use a Windows 7 machine as the server and Xbox 360s as extenders. This will allow you to have a shared guide, keep all storage on the server and utilize the InfiniTV with the least amount of hassle.
While it is true that you could house the InfiniTV in WHS2011 or Server 2008 R2, you will then using network bridging to share the tuners to other Win 7 client PCs. You will not have a shared guide/storage in this scenario and if recording CopyOnce, you will not be able to play the content on other clients beyond that which recorded it. There are many reasons why these limitations may not hinder some applications, but for your needs, the extender scenario described above seems like exactly what you are looking for.
One downside to using the extenders is you will need to pony up $50 (or is it $60 now…) to MS for Gold membership and use the dashboard for the UI which is highly unfortunate.