I'm not sure why I had an IR Cable Extender in the house, but I figured that it might help me control my Comcast DCH3416 HD Box.
The box is on a shelf down by hte floor, so to control it you have to point the remote down at the floor instead of at the tv.
I plugged one end of the IR Extender cable into the 'Serial - Service only' post, and then stuck the receiver on to the bottom right hand side of hte tv.
It worked as an IR extender.
Now I can point the remote at the TV.
Here's and IR extender on Amazon.com:
Infrared Receiver Extender Cable for HD DVR STB's