I have finally finished a long-planned but never-implemented feature of gensite, the software that builds this blog. As well as a simple chronological list of articles, you can now narrow your search using the new Tag Cloud.
If you select more than one tag, the articles that match all of your choices appear first. Articles that match only some selected tags appear down the list and are de-emphasized slightly. I think this is a fair compromise between being a strict logical AND, which is inflexible, and an loose OR, which doesn't filter enough to be useful.