I spent some time obsessively browsing http://mathoverflow.net during the last few days (and posting too), and it is a really awesome site. I found out about it last December, when I visited Berkeley to give a talk, and had lunch with one of the guys that runs the site, but didn't pay much attention to it until recently. It's much more suitable for discussion of research-level mathematics than general use and development of Sage.
Mathoverflow is interesting in that their comment system supports jsmath with realtime preview, which is something the TinyMCE editor in the Sage notebook doesn't do. It's also something that http://stackoverflow.com doesn't do, as far as I can tell. The grad students at Berkeley that organize mathoverflow probably somehow hacked jsmath into the comment system.
I was very surprised to learn that Mathoverflow and Stackoverflow (and all the dozens and dozens of stackoverflow-based sites) are closed source. There is a company that runs stackoverflow and hosts related sites, and keeping their code closed is part of their business model.
Some people sort of forked something related to stackexchange at some point, and created this: CNPROG, which is fortunately based on Python. There are about 20 sites using CNPROG now, including one for all questions about the use of Python in science, mathematics, and engineering.
I tried this site, answering a question about Cython, and it is snappy and clean.
Perhaps I will add jsmath or mathjax support to cnprog and start another site (http://q.sagemath.org ?), say on using math software as an aid to mathematical research. The site would not be specific to Sage or restricted to open source. The FAQ would say that all questions must be of the form: "(How) can I use a computer to compute XYZ, where XYZ should be some sort of advanced mathematical question." The emphasis on "advanced" and "research" is that there are already other sites about how to do elementary stuff (=people's homework), and it will provide an excuse to keep that out, which will greatly raise the quality. It is, after all, exactly this uncompromising focus on research that makes http://mathoverflow.net so interesting.
EDIT: I've now created http://ask.sagemath.org which is based on http://askbot.org, which is in turn based on CNPROG.