OK - so I saw this plugin today:
Cross Processing (Xpro) http://registry.gimp.org/node/25007
Which is based on this excellent tutorial by jEsuSdA: http://www.jesusda.com/blog/index.php?id=375
After running through the tutorial, I decided to write another version of the plug-in with a few options.
This plug-in requires Python. If you have the 'Filters->Python-Fu->Console' menu item you are all set. If not, install Python support for GIMP.
The plugin can be found under Filters>Artistic>Cross-Process. Don't forget to make the plugin executable.