So something in the webserver config has blown up since about 5:30pm last night HKT (5:30am EDT for the Americans). Since then pages have been sending blank headers to the fastcgi server and errors have been prolific. Bloody hell! Working on fixing this issue. Sorry for the crap.
UPDATE: 1:30pm I found a php-cgi.core file and fired up gdb. Looked like a problem with the filter.so for php, so I commented that out from extensions.ini. Everything seems far stabler from that point on.