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Wow - now I feel like my idea has been blessed by a god (Lead SEO at SEOMoz).....feeling quite chuffed actually!

I'm not sure you really have any sort of problem at all.  Unless you have seen a fall in traffic/ranking etc then I wouldn't worry.  DA calculations seem to lag what Google is doing anyway, so if you have not suffered in terms of traffic/ranking then I wouldn't worry at all.

DA is just an indicative metric about your domain, the end goal is conversions, so worry about it if those are hit but otherwise - stay calm and carry on (I should get that on a T-Shirt or something!)

Gary

I tend to agree with EGOL and Keri, I suspect you will be viewed as a spammer.

Yes, you are answering their questions, yes this makes for the internet being a warmer fuzzier place (and your answers might be the best damn answers in the world) BUT it will be clear to the forum owners that your intention is to generate a link.  The trouble is you also know that the reason you are doing this work is to generate that link.

I wouldn't necessarily not do it, but I would be prepared for quite a few of the links to be removed because they will be viewed as spam (even if directly on topic).  Don't get be wrong I don't think what you are doing is spam but it will be viewed by many forum admins as such.

Gary

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I would second that.  You also want to consider any other possible variations including things like /index.php or /index.html which might also resolve to your homepage.  All of these should be redirected to a single version to avoid duplicate content issues.