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I'm not aware of the actual effect of allowing link juice for banner ad links, but I assume Google is figuring out ways to devalue those links if they haven't already! If it's paid..I'd just add the nofollow to be safe!

If you figure out a way to do this, you'd be rich! They don't seem to care who they touch, whether it's Apple or SEOmoz of the little site. When I search Google for [negative keywords seomoz] I get back the post that I wrote about negative keywords, and Google has rewritten the title to be "negative-keywords-for-positive-roi - SEOmoz". negative-keywords-for-positive-roi is the URL/post slug! If you use different phrases to search, you'll find the correct post title. Quite frustrating!