There are some very early signs of a possible Google update brewing as of early this morning. A WebmasterWorld thread has some renewed chatter around an update.
Note, most of the WebmasterWorld thread is about April 15th changes, which people say have to do with the Boston bombings and seasonal traffic changes. But last night, early this morning, two webmasters came in and said they saw major shifts in rankings and traffic.
A preferred WebmasterWorld member said he saw a 77% drop just yesterday. Others said "some thing big is underway," after noting drops in his keyword ranking.
SERPs.com reports pretty significant changes in he Google results on Monday. SERPMetrics.com shows very little change in the search results. MozCast has not yet updated with results from Monday but they showed changes on Sunday, which seem off.
It is very early and nothing is confirmed - but there may be signs of a possible Google update. What exactly, is still unknown and unconfirmed.
Forum discussion at WebmasterWorld.
Update: Seems like there is now a wide spread update happening...