Because Digg Digg doesn't properly use WordPress's native wp_enqueue_script function and instead uses its own method to print scripts, the URL pointing to the Digg Digg js file is broken and thus the plugin does not work.
This is on a 3.1 Multisite install with the Domain Mapping plugin. Some notes: I don't have the ability to set a wildcard CNAME record in my DNS settings, and therefore the default URL for each site (subdomain of the root domain), is broken. If this were resolved the Digg Digg js URL would not be broken, but it would still point to the subdomain of the root instead of the current domain, which is incorrect. JS files should be included using wp_enqueue_script.