totalodds - 8:46 pm on Mar 9, 2013 (gmt 0)
I can confirm there was definitely an unannounced algorithm update on March 6th;
I've asked around, tweeted Matt Cutts etc but had no response.
It could be a dry run before Penguin or Panda. Both are overdue.
I'm not sure why Google are being so quiet, when the preach transparency.