I've rewritten the plugin to make use of the new extension mechanism in Safari 5. This means that the reliance on SIMBL is gone, and no private API's are used. Now it is just a webplugin-safari extension combination. The extension handles all the user interface, and the webplugin handles the actual validation.
Installation is pretty simple: copy the webplugin to ~/Library/Internet Plug-Ins, and double click the safariextz file. The latter requires you having enabled extensions in Safari. To do this, open the preferences, and go to the 'Advanced'-pane. Check 'Show Develop menu in menu bar'. In the newly visible 'Develop'-menu, select 'Enable Extensions'.
Please note that the W3C validation takes time! It will slow your browsing down, especially if the site contains a lot of (i)frames. Just go to the preferences, and disable the W3C validation.
HTML5 support isn't there yet, as I'm still investigating the best solution. The validator.nu engine (which W3C uses) assumes you will be running it as a webservice, which is not ideal in a browser situation. Opening a port and running a server in the browser is definitely a worst case solution.