I'm not sure that there is much point to this...spam.module is MUCH more effective at making this not a problem. And what if you WANT user submitted content to count towards pagerank? Like, say, people in forums constantly pointing to the same RTFM post.
i see a much point. maintaining spam.module is much harder than just rewriting some links. the idea is not to rewrite internal links (such as your forum example), but rather to rewrite external links.
For external pages, it is just the same, you might just be happy with increasing some page's pagerank. On some forums, users tend to suggest others going to http://www.catb.org/~esr/faqs/smart-questions.html or the native language translation of that before asking more questions :) Now it would clearly be positive, if these type of pages pagerank would raise :) Goba