Try this:
I could not play purchased HD TV shows and movies from iTunes anymore! I finally figured it out to some extent, by that I mean I'm pretty sure this fixed my problem, though I'm not exactly sure why...yet.
I was, silly me, running itunes in 64 bit mode in ML. In order to get HD content to play on my hdmi connected Panasonic HD TV I have to run iTunes in 32 bit mode.
To run itunes in 32-bit mode, select the itunes application in the Finder, choose the "get info" command int he file menu, and check the "open in 32-bit mode" checkbox in the itunes info window.".....bada bing!
Not sure what kind of repercussions this might have yet but I can now watch Game of Thrones in HD! Hope this helps you all.