I've thought about this many times. How can you create a system that preserves the anonymity of publishers in persecution situations but excludes the child pornographers? The difficulty comes down to the fact that there is no technical difference between the two classes of publisher, just a moral one. Suppose the persecuted wants to share pictures of children being abused by government forces?
One idea I had was for the system to be semi-anonymous. Publishers would form public groups and the publication of content comes from the group as a collective. The members of each group are known, but the specific originator of the content within the group is not. This is the spartacus model of anonymity :)