Considering you can get a Windows-based HDTV tuner card for around $300, that seems way high...
here are a few I've located:
MIT MDP-100
HiDTV
HiPix DTV-200
AccessDTV Digital Media Receiver 2.0
Win-TV HD
MyHD MDP-100
I have to wonder what the point of advertising the Apple 23" LCD Cinema display as "HD" when there doesn't seem to be anyway of actually showing HDTV on it.
The only way I can figure out so far is to buy a stand-alone outboard HDTV Tuner with DVI output, and then buy an ADC to DVI converter and connect it that way. Even then, I'd be skeptical that it would work.
If someone has a better solution speak now or forever hold your peace!