As long distance photographs are either entirely in focus or entirely out of focus, the only effect the shift lens is having is to apply a variable blur. Therefore, we can achieve the same effect in post processing, for example by using this variable blur plug-in for the GIMP.
Original photo:
Blur map (a simple bi-linear gradient):
Result:
(I threw in a little "Filters/Noise/Scatter RGB..." too, to simulate film grain.)
Note: To achieve this effect "properly" (that is, get the depth-blurring effect exactly correct) you would have to build a camera with an enormous aperture. The shift lens is just as "fake" an effect as what I did above.
Here's a couple more:
