It is a WPMU with BuddyPress which I set up a few months ago. It is still only a test site, so not live with lots users (who be a bit fed up by now).
I have the domain mapping plugin from wpmu installed, though none of my subdomains are being mapped at present. The problem is simply with pure subdomains on existing blogs created while previous version of BP was installed. And on new blogs created too.
Problem only exists while BuddYpress 1.5 is activated. I deactivate BP and all my subdomains are back.
