This script generates two difference layers from the two layers on
the top, A is the original image layer, B the modified (e.g. sharpened)
one on top:
It's similar to GIMP's built-in Grain Extract/Merge function, but the tonal range is bigger.
Copy it into the scripts folder from GIMP, you will find it then under Layer -> Adjustment Layers:
Finally a big thank you to Shelly and Roy from the Flickr GIMP group:
Update1: I renamed the script from "Adjustment" to "Difference", which describes the script's function better.