g1smd - 7:18 am on Sep 6, 2011 (gmt 0)
Serving a "site temporarily offline for updating" message with "200 OK" with or without 301 redirecting all site URLs to an error page, is a big bad idea.
DNS failure, server meltdown, etc will just timeout and return no website. Serving "can't connect to database" with "200 OK" is asking for trouble; serving 503 is much better. No idea if there is a definitive list.