Not to undercut my own argument for why a StackExchange site would be nice, but this already exists:

And, as far as it goes, this isn't very difficult to do:

However, the value to me is that nobody has to write any code for us to use StackOverflow, people are already using it, it reaches an audience outside the normal *.drupal.org sphere, and it will keep getting better as the general StackExchange platform gets better.

If it really takes off and people decide to build a viable clone on something.drupal.org, we can easily migrate the Q&A data. They make it all available as dumps or via API and user contributions are Creative Commons licensed:

-j