Just wanted to throw out another data point. I just recently got my HD-PVR working with MythTV, and an SA4250 and it works very well. With the new NVidia drivers and MythTV allowing video decoding to happen in the GPU hardware, the CPU usage is MUCH better. Before I used to hit about 70% utilization playing back OTA HD MPEG2 recordings, which don't really require a lot of decoding horsepower, now with VDPAU support in MythTV I can playback H.264 recordings from the HD-PVR with about 15% CPU utilization.
I just upgraded to MythDora 10.21 and it includes drivers for the HD-PVR already, so I didn't even have to do anything, it just worked.