Just to throw in my 2 cents.
I have my HTL-HD hooked up to a 46" Toshiba HD rear projection set. I have noticed none of the degradation on SD that others have reported. It is comparable to output from my SD Tivo through composite cables.
I also have coax output run to a TV in a bedroom that IS ACTIVE at the same time as the DVI output. The switch controls component/DVI output. I think I have tried to the composite output to see if it is active with DVI, and I think that is was active at all times as well. So to sum up, I think that coax and composite are active at all times, the switch controls the component/DVI output only. I'm sure I'll be corrected if I'm incorrect.