As of yet, there isn't a patch here because we first need to figure out what exactly is needed, preferences, etc.
First off, reference google's announcement of rel="nofollow" and what it does: http://googleblog.blogspot.com/2005/01/preventing-comment-spam.html
This patch should be compatible with /RobotsMetaTag, as that should become part of the standard usemod soon.
As for preferences, it might be sufficient at first to have a single regular expression preference value which external links would need to match in order to not be marked nofollow. (We need a verb for what's happened to these links - "depageranked"?) Of course, also a preference value to turn on depageranking at all, by default on.
See MeatBall:NoFollow for dissent.