[Stuart]

The problem is caused by the way the F1 coverage is shot. While all the channels covering it send commentry and sound teams out to each race, the video is provided by the local channel (so, the BBC would provide coverage for the British Grand Prix, a US local channel the coverage for the American Grand Prix).

The problem is caused because not all the local channels have HD equipment and facilities, and the FIA likes coverage to be uniform. The same happened when Nicam was introduced, and the same with Widescreen broadcasting.

Of course, that wouldn't prevent the BBC filming the British Grand Prix in both HD and SD, but there are presumably other reasons why they are not doing that..