You can already do this with a timing attack. Use js to add a script tag with http://facebook.com/js/somefakefile.js http://facebook.com/js/somefakefile.js to

You can already do this with a timing attack. Use js to add a script tag with http://facebook.com/js/somefakefile.js http://facebook.com/js/somefakefile.js to the page, and time how long it takes to load. If it's in cache it will be much faster.