Why did Bill Keller Resign as The New York Times' Editor?

The announcement today that Bill Keller would be replaced by Jill Abramson as Executive Editor at the New York Times was something of a bombshell. Keller, it seems, has been achieving a higher and higher profile lately with his columns for the Times. So what’s the story behind this? The official statement is that Keller stepped down, and his boss Arthur Sulzberger Jr., the paper’s publisher, accepted his resignation “with mixed emotions.” But is there more to it than that?