"It’s been about two weeks since Google launched its Penguin Update. Google’s happy the new spam-fighting algorithm is improving things as intended."
From the original article by Danny Sullivan at Search Engine Land some key points I extracted:
"...Penguin, like Panda, is a filter that gets refreshed from time-to-time. Penguin is not constantly running but rather is used to tag things as spam above-and-beyond Google’s regular spam filtering on a periodic basis.
...To further confuse matters, some who lost traffic because of Penguin might not be victims of a penalty at all. Rather, Google may have stopped allowing some links to pass credit, if they were deemed to be part of some attempt to just manipulate rankings. If sites were heavily dependent on these artificial links, they’d see a drop just because the link credit was pulled, not because they were hit with a penalty.
...if you know that you were hit by Penguin (because your traffic dropped on April 24):
-> Clean up on-page spam you know you’ve done
-> Clean up bad links you know you’re been involved with, as best you can
-> Wait for news of a future Penguin Update and see if you recover after it happens
-> If it doesn’t, try further cleaning or consider starting over with a fresh site
-> If you really believe you were a false positive, file a report as explained here
Just in, by the way, a list of WordPress plug-ins that apparently insert hidden links. If you use some of these, and they have inserted hidden links, that could have caused a penalty."
(Suggested by Giuseppe Mauriello)