Well, I've spent quite a long time now playing around with different filters and effects and I've knocked together this quick Qt application to try them all out. I'm quite pleased with the results!

As an added bonus, although I developed my Qt test app on a desktop machine it cross-compiles to N900 with no modifications. I love Qt! If anyone thinks it's useful, I'll develop this into a releasable application that can be used to apply classic photo effects to existing photos. Let me know what you think.