Wayne Lipe...
Sharpening should always be done last in any post-processing and it is recommended to shoot with minimum contrast and saturation if you post process the images.Some people think that any image which has been post-processed in the `digital darkroom' (Photoshop, GIMP, etc.) should be openly declared as `manipulated'. For those of you who would like to know when they can claim whether an image has not been manipulated, take a look at this Photo.net page.
GimpGuru -- Lots of handy photo manipulation tutorials for GIMP users.