I agree with most of the posts on this thread.
There are two levels for the BBB:
1) local/regional
2) website/national
We almost purchased the web based BBB package (which, I believe at the time, made it mandatory we joined locally,) just to get the BBB seal on our website, which, again, at the time, was supposed to give us a bump in our online conversion.
Well, we never purchased either, so I'm not sure if it helped or hurt, but I have yet (in two years) have anyone ask us if we're a member. And there are a ton of other credit-building links for your website.
Good luck,
Chris