There was scuttlebutt about this from different sources, but now it's official.
The Simpsons showrunner Al Jean just issued a statement confirming that original voice cast member Harry Shearer is leaving the show. “Harry Shearer was offered the same deal the rest of the cast accepted, and passed,” Jean said in a statement released by series producer 20th Century Fox TV. “The show will go on and we wish him well. Maggie took it hard.” He indicated that the characters voiced by Shearer, including Burns and Flanders, won’t be killed off, but the roles will be recast. ...
Studio insiders told me that Shearer was the last voice actor to be resigned, that he had reached an agreement just before the first Simpsons table read, but that information was in error.
One of the gossip sites (TMZ) had subsequently reported that Hearer had NOT signed, and it's now clear that information was correct. Harry, per the article, wouldn't agree to a pre-determined schedule, and so ... adios.
He's a talented, talented man, and has many creative fires burning at different locations on the globe, and I'm confident he'll get along splendidly without the Simpsons gig.
Add On: The Shearer Saga continues:
... "(W)e're still hoping he might come back," said Al Jean to Dan Snierson of EW.com. "Nothing's done 'til it's done." ...