How to retroactively apply Mollom spam filtering? I've been asked to clean up a site that's been running for a few years, and has accumulated some spam in addition to a lot of valid content.

I've just installed Mollom on the site to help reduce future spam postings; but is there some way I can have Mollom process what's already been posted to the site, and flag potentially-spammy content for my review (rather than having to review all of the mostly-valid content individually)?
Not currently. See http://drupal.org/node/734476 for the feature request in the Mollom issue queue.