LinuxMCE does support the Hauppauge HD-PVR. If your cable/satellite box supports component output at 720p/1080i, then you can connect your cable box to this tuner, and use it this way.
However, do keep in mind too, that LinuxMCE, can not only control your cable box, but it can understand how the cable box is connected to your AV gear, so it can send commands to your TV, Amplifier, etc, to switch the inputs to the cable box, if you choose to use the cable box directly, as well as digitize the cable box to send through the rest of the house, if so desired, (MythTV also supports this.)
-Thom