The choice about whether or not to encrypt traffic should be left to each website's administrator. Many sites--shock!--use the web to disseminate information they wish to be public, and the site's users have no problem with their access to it being public either. So get out of their faces! Using the browser to deprecate admin's particular choices is contrary to the spirit of the web, which should always do its damndest to serve something, and degrade gracefully when it's in difficulty, not pop annoying dialog boxes in the user's way.
Self-certificates are already a fairly effective denial of service attack when Firefox is used to access many independent sites that try to implement https, but who fail to do so in a way that offers a smooth user experience to J. Random User (I'm thinking particularly of IndyMedia).
Please note: in China, the censorship does not rely on blocking everything; just on blocking enough that all but the very motivated fail to access it. This troublesome minority can presumably be picked off at leisure later.
Keep it simple, stupid!