From the Clinton campaign:

Statement from Maggie Williams
 
After the events of the last few days, Mark Penn has asked to give up his role as Chief Strategist of the Clinton Campaign; Mark, and Penn, Schoen and Berland Associates, Inc. will continue to provide polling and advice to the campaign.
 
Geoff Garin and Howard Wolfson will coordinate the campaign's strategic message team going forward.

Questions: Was he pushed or did he jump? (Is it possible Penn decided Hillary's chances are too slim to keep sacrificing his corporate work?) What does this mean for the campaign's strategy? (Penn was always a proponent of leadership and experience themes, Wolfson has historically been more keen to humanizer her.)

And here's some background on Garin (who first joined the Clinton campaign in mid-March):

In politics, Mr. Garin has a well-earned reputation for helping candidates win in difficult circumstances. In 2001, Mr. Garin’s strategic research helped Mark Warner win the governorship in Virginia, despite the state’s strong Republican leanings. Mr. Garin has directed the polling and created winning campaign strategies for many of the leading Democrats serving in the U.S. Senate, including Dick Durbin, Dianne Feinstein, Chuck Schumer, Russ Feingold, Robert C. Byrd, Jay Rockefeller, Patrick Leahy, Max Baucus, Kent Conrad, and Byron Dorgan.  In the 2004 presidential election, Mr. Garin was the polling advisor to General Wesley Clark and was the pollster for the Democratic National Committee’s independent expenditure campaign in the general election

--Michael Crowley