Was just browsing around looking for wireless HDMI transmitters and found these...
http://www.aavara.com/en/products/hdmi.php?id=15
Looked around further and found a few places selling them. Sure ain't cheap but could be just the ticket for someone with a house pre-wired with coax only and who can't run any new cables.
Had thought of the concept a few years ago and thought it'd be a good idea but didn't find anything last time I searched. Guess someone else thought it was a good idea too :)