One of the authors over at http://www.nilkanth.com/ has given us this great article outlining how they’ve developed an easy 12-step process for achieving the Tilt-Shift Illusion:
1. Play with the photo’s color curves, brightness and contrast
2. Activate Quick Mask
3. Reset colors in toolbox with the ‘d’ key (so that FG = black and BG = white)
4. Select Blend (Gradient) tool
5. Set Gradient to FG to BG (RGB)
6. Set Shape to Bi-linear
7. Think of a focal point in the photo (i.e. where the key subject is based)
8. Use the Blend tool from your focal point on the Quick Mask (you’ll have to play around with this a bit to get the right size/shape)
9. Deactivate Quick Mask (you’ll see the selection outline now)
10. Apply Gaussian Blur (or Focus Blur for an even better outcome)
11. Invert the selection
12. Sharpen (Enhance) the inverted selection
You’d have to be fairly conversant in the use of GIMP to realise the gist of this article. It’s easy enough to execute, but needs refining. Step  has an interesting suggestion to ‘think‘ of a focal point. Just follow through, it makes sense why they say this, even if it is a little amusing in the methodology of these instructions.
I plan to test this out over the weekend. Should it be necessary, I’ll rewrite this doco’ with an easier explanation. I usually find easier steps to achieve an image, so probably will do a new documentation anyway.
Whilst researching for this article, I also found Jacob Wighton’s method for tilt-shift creation with GIMP!