I finally got fed up enough with WordPress.com’s blurry image-based maths rendering to write a bookmarklet which goes through the page replacing them with the equivalent MathJax code. I’ve added it to my MathJax bookmarklet installation page. Drag the “image replacement bookmarklet” to your bookmarks bar, go to a WordPress.com blog (I recommend Gowers’s Weblog), then click the “Replace image maths with MathJax” bookmarklet.
Update 19/09/2013: Following a suggestion by Peter Krautzberger, I’ve updated the bookmarklet to do the image replacement differently, so the images don’t disappear while MathJax is working. I’ve also made it work on Wikipedia.