As a spinoff of the discussion of category intersections on wikitech-l, I have ported my toolserver-powered JavaScript add-on to Commons.
Screenshot: http://commons.wikimedia.org/wiki/Image:Category_intersection_screenshot.png
The tool will add a new link beneath the category list of a page. A click will open the list of categories of this page(/image) as checkboxes. Chose two or more of these, and you can preview 20 files in that intersection, or go to the toolserver to see them all.
To use, add importScript('User:Magnus_Manske/category_intersection.js'); to your monobook.js. We can turn it into a gadget if wanted.
Cheers, Magnus