Installing the Movable Type “nofollow” plugin from Six Apart will automatically update the built-in tags MTCommentBody, MTCommentAuthorLink and the container tag MTPings. The use of the MTPings container should serve to update any URLs contained within, such as those produced by the MTPingURL tag and surrounding template code.
Note that MTPingURL is in use in the default templates (in the Individual Entry Archive and the Trackback Listing Template). In case the MTPings container method doesn’t work (and there is no reason that it shouldn’t), then you’ll need to adjust these templates manually. Nothing appears to happen with MTCommentURL, which would make sense, as it simply displays a URL (no anchor text around it to update). In case you use this tag, you’re on your own to manually add the rel=”nofollow” text to the link around the tag, if it is in use in your templates. Keep this in mind!
You do have to rebuild your site after installing this plugin, if you are using static templates. There appears to be a PHP version, so if you use dynamic templates, you should not need to rebuild. It appears that the plugin will only execute if you are running a version less than 3.2, implying that Movable Type versions 3.2 and higher will include this by default, and not need the plugin to do it.
Users of Six Apart’s hosted services, LiveJournal and TypePad, have the issue addressed for them, so that this attribute will be added immediately to all accounts. It might be a short wait, but apparently it will be in effect very soon.