Fox News Channel says that this information comes to us via a senior administration official. Apparently, he called the president to say that he is done.
The New York Times has more:
Attorney General Alberto R. Gonzales, whose tenure has been marred by controversy and accusations of perjury before Congress, has resigned. A senior administration official said he would announce the decision later this morning in Washington.
Mr. Gonzales, who had rebuffed calls for his resignation, submitted his to President Bush by telephone on Friday, the official said. His decision was not immediately announced, the official added, until after the president invited him and his wife to lunch at his ranch near here.
Mr. Bush has not yet chosen a replacement but will not leave the position open long, the official said, speaking on condition of anonymity because the Attorney General’s resignation had not yet been made public.
Mr. Bush had repeatedly stood by Mr. Gonzales, an old friend and colleague from Texas, even as he faced increasing scrutiny for his leadership of the Justice Department, including his role in the dismissals of nine United States attorneys late last year and questions about whether he testified truthfully about the National Security Agency’s surveillance programs.
There have been a lot of rumors that Chertoff will replace him.
Press conference has been scheduled for this morning at 10:30am.
Update: Don’t forget that just a couple weeks ago, when Karl Rove announced his resignation, we learned that Bush’s Chief of Staff Josh Bolten said that those who weren’t prepared to stay for the remainder of Bush’s term should leave by Labor Day. Gonzales’ resignation seems to fall in line more with this than with any pressure from critics for him to leave