Quick notes in case it helps some random googler, or me later when I forget what I did...
I finally bought a Hauppauge HDPVR to go with our mythtv system. It's hard not to HD once you know it's there.
Installed using mostly the HDPVR page on the mythtv wiki.
Our Rogers Ottawa Scientific Atlanta SA4250HD might support firewire, but I don't have firewire on my machine, so I kept going with the MCE IR blaster to change channels.
Unfortunately every channel change would hang the HDPVR. After some attempts to figure that out I pressed the distribution upgrade button and went to mythbuntu 10.10 from 10.04. Magically things started to work.
I could now record HD video, but NVIDIA VDPAU playback had stuttery audio through Pulseaudio. A little comment about VDPAU & cpu frequency scaling caught my eye on the VDPAU wiki page and with a little fiddling I changed my lower limit to something that worked.
Not sure why, but I had to "sudo su -" to be able to enter this command:
echo 2700000 > /sys/devices/system/cpu/cpu0/cpufreq/scaling_min_freq
Before I changed it the minimum frequency on my machine was at 1GHz. Seemed like a good idea when I set it to save power, but apparently it slows down a bus somewhere and doesn't allow the GPU to communicate fast enough.
That worked until the next reboot. I'll have to figure out how to get it to stick another day, probably when I figure out how I'm really supposed to use cpufreq/ondemand.