Hmm I watched the video, can't help but think how pointless it is! Firstly, there's absolutely no benefit to be gained in SEO terms from dupeing content - even more so if you're doing it from an authority like Yahoo! Secondly, anyone looking at those posts immediately knows they're scraped because of the "Powered by Yahoo Answers" text.
Because of the lack of any SEO benefit, these threads aren't ever going to show up in any search results, so there's no worries about spam appearing IMO. As for giving your forum a head start... if I saw a forum like that I'd just think "this yahoo answers looks worth a go...!!"
I can see the point of using it to a certain extent to provide your visitors with extra info, but I'd be very cautious of driving them away to Yahoo because the info was so good!
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