My usual response is "try it, then ask me again".
With utmost respect to Matt and big love for Wordpress, I just don't think protectionism makes sense here. Wordpress.org and .com will surely have unbeatable search rankings and a solid enough SEM campaigns to remove any doubts about what is and is not official Wordpress content? And the "Would the website be any less effective if it was wpforgood" question applies in reverse, does it not? People don't pay that much attention to domain names and could still be hoodwinked by a well put together site.
Good work Dave, signing up now. As well as this being dead useful as a hub for all things barcamp, it will be interesting to get a feel for Ning too having not yet used it myself. I wonder, did you consider / have you previously tried alternatives like socialgo?