Is the software used to run stackexchange open source? I haven't found any info on this in http://en.wikipedia.org/wiki/Stackexchange or http://stackexchange.com/about . A related question is the status of its data. I understand that everything is CC-BY-SA licensed (which is great!), and that, e.g., mathoverflow posts dumps periodically at http://dumps.mathoverflow.net/ . If SE turned out to be unprofitable and Union Square Ventures didn't come through with more money, could sites like physics.se have their data preserved and be rebuilt with different hosting, or could everything just vanish into a black hole? If the software is proprietary, would such a rebuilding involve recreating all the software from scratch?
EDIT: Found this http://www.readwriteweb.com/start/2009/10/stackoverflow-shares-its-mojo.php , which seems to indicate that the software is being sold, although that doesn't necessarily mean it's closed-source (e.g., Red Hat sells a flavor of linux that's open source).