penders - 10:36 pm on Jan 8, 2012 (gmt 0)

Maybe you could fetch the referring page using CURL and validate it by making sure that the reference to your site is not in one of these known 'cheating' code blocks? Or maybe it is sufficient to just make sure that the link to your site does not appear within a src attribute - presumably it should only occur within an href attribute of an anchor? You could also penalise them if they have included the rel="nofollow" within the anchor?