This solution may be self-evident, but I couldn't find any information on how to get my Mythtv setup, installed via Knoppmyth, to feed 1366x768 to my Panasonic ph509uk through the dvi blade. Most of what I read involved correcting EDID settings or coming up with a custom modeline. It turns out that EDID is sometimes true, but my xorg.conf file had set it to false. So all I did was to change xorg.conf from (going on memory here, so maybe not exact) "EdidDpi" "FALSE" to "EdidDpi" "TRUE" and inserted "1366x768" at the beginning of the 24 bit depth and 32 bit depth lines specifying resolution, e.g.:

SubSection "Display"
Depth 24
Modes "1366x768" "1024x768"

It seems that the default was set to false, so before you go changing EDID settings or trying to figure out your own modelines, you might check to see whether your EDID file might be telling the truth.

Two other points are worth noting: I have an nvidia 7600, which is apparently now liberated from the multiples of 8 limitation even in linux. Second, previously I had to adjust the aspect ratio in Mythtv manually when watching TV; at first the correct aspect ratio would be detected, then change to an improper ratio, and I would have to reset it. Now the opposite happens: An incorrect aspect ration displays for about a second and then is corrected automatically. Hope this helps someone looking for resolution.