The way it's designed to work, is this:
* LinuxMCE can control A/V devices in your setup, including cable boxes.. in the case of cable boxes, this is done typically with infra-red, using a USB UIRT and a Xantech 286 double emitter.....
you connect the cable box directly to your TV and amplifier, as normally.... using the high definition outputs....
then, you connect the low-def outputs (s-video and l/r audio), to tuner card, a PVR-150/250/500/USB2.
MythTV will use the PVR card, and record, if wanted....
you also gain a non-pluto TV source, which talks directly to your cable box, and sends its output directly into the tuner card and out to the TV... any remote presses go to your cable box automatically (this is the first level of A/V control)
if you press Direct A/V in that menu, it will switch inputs on your TV and amplifier as needed, so you get the HD signal from the cable box.. remote presses still go to the cable box, and if a menu needs to be displayed, the inputs will be flipped back to the tuner card.
The hauppauge HD-PVR 1212 will be supported soon as an option here so that HD content can be captured and streamed in this manner.
Also, the HDHomeRun can be used to get OTA HD and in-the-clear QAM channels on your cable box, in great quality.