Not sure if this will help, but here's what I've done... I redid my xorg.conf again without dual head/twinview/etc... I used the "IgnoreDisplayDevices" option as such:
Option "IgnoreDisplayDevices" "DFP-0"
Now, I no longer get the horizontal descending tear.
However, this is obviously not a good solution since I would have to make changes to/swap xorg.conf files whenever I need to use the built-in screen.
I am hoping, though, that maybe this points us into a direction where we can come up with a real solution.
New xorg.conf: http://landlordhat.com/xorg.conf.090427.txt