I'm wondering if there exists any program to integrate with the N900's camera and save two copies of the captured picture - one to the normal Photos directory and another to a hidden directory that can only be accessed via the file manager (or maybe even by taking out the SDHC card and plugging it into a different device).

Seems to me it would be useful for all the people who get told photography is a crime when they try to take pictures of buildings and monuments and such. They'd be able to "delete" the picture but still keep it and plausibly deny that it exists.