I have both my PC and a laptop plugged into HDMI, the laptop is a sound and video interface, but for my PC I bought a DVI to HDMI and use the optical line out for sound.
For Video however, the HDMI device just identifies it's self as another monitor, so I can change resolution, refresh rates, bit depths etc etc as I please. It (the nVidia driver) also allows you to select either screen as primary, both screens cloned or extended desktop.
for watching 1080p media, nothing beats this set-up. I do have all less than 1year old hardware and don't use any 'essential' software junk, I like a very clean PC so do all my video adjusting with nothing more than the nVidia drivers.