On another forum I frequent, someone posted a link to a fake tilt-shift video and asked how it was done.
Previously I had seen still tilt-shift images and googled around to figure out how to create them using Gimp. Lots of references like this one at Gimparoo, or variousblogs. One thing in common these had was using a gradient mask to blend a blurred copy with a non-blurred copy, or in the case of fancier efforts that use the focal blur plugin, a gradient to drive that plugin.
To me, these efforts left a bit to be desired, as the gradient would leave odd artifacts of focused bits of objects crossing in front of the focal plane gradient point. I figured a good way to get around this would be to construct a complete mock depth map based on the image. Here is an example of a before image, and the fake tilt shift.