WinTV HVR-850 finally working in Linux
That was more painful than it needed to be.
The key to getting the HVR-850 (or HVR-950q, seem to be the same hardware) to work moderately well was placing the following in /etc/modprobe.d/xc5000.conf :
options xc5000 no_poweroff=1
This seems to allow the HVR-850 to do work without reloading itself randomly. I'm using a stock Debian 2.6.32-4 kernel (from unstable at the moment) and the dvb-fe-xc5000-1.6.114.fw firmware file. This loads the "xc5000" kernel module at startup and picks up the firmware from /lib/firmware without issue.
Another useful bit of information is that MythTV won't scan the ATSC channels properly unless the timeout was set to be 6 seconds or so (where the DVB card is configured in MythTV there's 2 places enter these timeouts as 6000 ms... set them large or Myth won't work in the Scan step).
Now that this has been done, I'm pleasantly surprised at how much info comes over EIT (the list of upcoming programs) ... so much so that I may not bother with schedulesdirect.org (though CityTV and OMNI really should start putting some usable info in their streams; I'm curious why they wouldn't). I'm unhappily surprised at how much worse the tuner is than the Digiwave/Electronic Master DTV-5000 set top box I have. It doesn't bring in nearly as many channels and the weaker of those it does seem intermittent. I've been considering using an HDHomeRun as an alternative. Doing so would also likely avoid the hassles I had with USB (not every USB 2.0 card likes the HVR-850) and also allow me to pick up more channels (as it seems the tuner in the HDHomeRun is much better than the WinTV tuners), and more channels simultaneously (since it has two tuners built-in).
I'm watching the results of these recordings on an HDX-1000 (through component video to my older Toshiba 34" CRT which still works well) as none of the computers I have are fast enough to decode & display the HD mpg stream. They do seem to display well, in the end.