but I won't belabour the point :)
Actually I will make one more comment (sorry) :) because I do actually have sound reasoning behind my suggestion beyond just "it's better", and it is only right I lay them out.
(I've maintained/operated/implemented a number of Q&A sites for small communities and various businesses so this is my experience)
If you stick OSQA up on help.wikimedia.org then very little will happen - it is unlikely to turn into the go-to help site for Wikimedia projects, you'll probably get an initial rush of contribution and then it will tail off as the questions stack up and the volunteers who are left to answer start to become overwhelmed.
The part of our audience that would need the Q&A is not the same as, say, OpenStreetMap. There you have people using the tool that need a forum to ask for help and advice. This we already have in a fairly effective form; discussion and project pages - and the people needing that Q&A will get better help that way.
In addition any self-hosted narrow focused site is unlikely to get a good seed of people able to provide support/answers (take this advice from experience :)).
So why would a hosted Q&A work better? Well because you could allow the scope to be expanded; a lot of the help needed is general to *any* wiki so it opens up any useful content to a wider audience. And such a site could still handle more specific queries.
But the main advantage is that your putting it under a name and community who are already experienced at doing really good QA - so your seed of volunteers is going to be that much better! You will get SE veterans who are also Wiki editors that will be much more inclined to contribute, for example. With a site such as this, kudos and points means everything - because answering questions (especially the horribly mundane ones..) is tedious and boring work. And SE have nailed that vibe.
I understand the issue of not having such a site under our control. But at the end of the day, doing Q&A is not our ball game. And we can end up with a much better service for our own users by biting the bullet and admitting it :)
There; hopefully that makes a little more sense!
I think a Q&A site is definitely something really important to explore as a way to encourage more contribution and a better understanding of Wikipedia and how it works.
Tom