Displaying computer output on an HDTV is already possible – if your computer is near your TV and you have the right kind of video card or adapter. Or you can use a dedicated device such as the Wherever TV receiver, that uses the wherever.tv site to mirror channels from all over the world (but still needs to be near your TV).
Or you can use ZeeVee.
ZeeVee takes a very clever approach that makes it possible to display the output of any computer to any TV in your house without any rewiring on the TV side. The ZeeVee box connects to the VGA output of your TV and to your cable system. It then takes the computer output and rebroadcasts it on an open channel on your cable system. A USB remote control receiver allows you to control the computer output from wireless remote controls throughout your home. You do need to add a filter to the cable input of your house to prevent your computer output from being rebroadcast through the neighborhood.
The ZeeVee box is designed primarily for cable TV systems. Whether it works or not on antenna or satellite systems depends greatly on the system and the TV. You also have to have a computer running XP SP2 or Vista with VGA output (some newer custom made computers may have DVI output only).