Yes 99.9% of HDMI video cards support HDCP provided it is connected directly not via a converter, I have two HDMI video cards (ATI Radeon HD 4800 Series) connected to a HP w2448hc HDMI Monitor, the only headache I have had so far is with the HDMI drivers from both ATI and Realtek HD Audio and this relates to who is going to take charge of the sound coming from the HDMI Cable, I found that if you install the ATI driver and restart your computer you end up with an unaudible sound but install Realtek HDMI driver straight after makes that sound go away, anyway coming back to the first question "
Is it possible to connect a vista media center to a HD set top box receiver and then connect the media center to a tv again via HDMI?" Yes it is possible but MCE don't have the plug-in to run native HDMI but AverMedia do (AverMedia AverTV CaptureHD Hybrid TV Tuner) the only downside is that you have to run the Aver MediaCentre which is not a bad thing, this is the card that I run along with an HD Set Top Box from Sky TV running in harmony with each other.
[IMG]file:///C:/Users/KiwiSA/AppData/Local/Temp/moz-screenshot.png[/IMG]You can go to
AVerMedia AVerTV Global - AVerTV TV Tuners and Multimedia Products - AVerTV CaptureHD to get a run down on the card, it works in full HD and works well.