I don't really know if there is such thing as 'too much' post-processing. Take a look at
Jerry Uelsmann's work.
Some things I wouldn't do post-processing on, but I had the most fun in my photography class at the end of the year when we got to do special effects. It was sort of like telling us that we'd learned how to use the camera to get the shots, and the darkroom to print the shots, and now we could truly unleash our creativity and do what we wanted.
edit: Some of Mr. Uelsmann's work is NSFW - artistic nudes.