There are two products that have come to my attention that can come in handy for people trying to distribute HD content over the coax as a Digital cable (QAM) channel inserted onto any empty channel.  The products are by a company called Zeevee.  The first product is called the ZvBox 100 bundle which includes:

• One ZvBox™ – build your own HDTV channel and localcast your computer to all your HDTVs
• One ZvRemote™ – Control your computer from anywhere in the home
• One ZvReceiver™ – Connects the ZvRemote to your computer
• One VGA cable – Video connection from your computer to the ZvBox
• One USB cable – Digital Audio from your computer to the ZvBox
• Four 2 meter COAX cables – Helps you connect ZvBox and your TVs to your cable
• Two cable splitters – Helps you tap into the cable wiring of your home

This allows you to take any video from a VGA source (say a computer monitor output) and broadcast it via coax to any empty digital cable channel.  The remote allows you to control your computer so even if the source is in a remote part of your home, you can still control the mouse as if you were locally.   The ZeeVee ZvBox 100 Bundle
sells for $399.

The ZvBox 150 has the same capabilities as the ZvBox 100 but it also adds the ability to plug in any component HD video source as well.  You can combine many ZvBoxes to distribute many HD channels in your location, on the same simple coax cabling.  Once configured, the ZvBox 150, no computer or Internet connection required.  A quick google search reveals that the ZvBox 150 is priced higher than the ZvBox 100, at around $1000.

Depending on your situation these two products can be the answer all your HD cable distribution/modulation needs — definitely check these products out!