The famous Digg This plugin for wordpress just got updated (thanks for thetshirtblog from the forum for notifying me of the problem).

As it turns out that Digg changed the HTML code consist of a post. Since the plugin validate that the referral you got is actually from digg and not from a spam bot, it did not parse correctly the changed HTML coming from digg, resulting no link back to digg. If you have an old version of the plugin, please update your wordpress with the new version