The first one was Panda released in February and then the international rollout in April, where some SEOs saw incremental returns. And now, early May, webmasters and SEOs are saying Panda has shifted a bit.
One webmaster said:
Is it possible that Panda 3.0 just rolled out (as of May 6, 2011)? Perhaps it's in the process of rolling out? I'm seeing a massive shake up in the search niches that I monitor, results are bouncing all over the place. My websites seem to be slipping again to a new all time Google referral low.