The Washington Post's Chris Cilizza has the scoop:

Illinois Attorney General Lisa Madigan (D) will run for re-election to her current post in 2010, according to a Democratic source aware of the decision, ending months of speculation about whether she might make the leap to a Senate or gubernatorial bid.

Madigan is expected to make her decision public later today.

Wow.  Get ready for the floodgates to open on both the Senate and governor's races. 

UPDATE (10:05 a.m.): Lynn Sweet confirms it.  And Madigan's campaign operation just sent out a press release announcing a 2 p.m. press conference in downtown Chicago.

UPDATE II (11:00 a.m.): So with Madigan out of the Senate race, what does the Democratic primary field look like?  Here are the four big names in the mix:

Roland Burris: While he and consultant Delmarie Cobb are giving every indication that they plan to go forward with a 2010 campaign, numerous polls this spring showed that his approval rating is in the dumps and a vast majority of Illinois Democrats don't want him to run.  

Alexi Giannoulias: Currently the state treasurer, his exploratory committee has been running full bore for several months now, after raising an astounding $1.1 million in the first quarter.  There has been an ongoing debate over whether he would stay in the race if Madigan opted in, but that's all moot now.  Having run for statewide office in the past, he has solid name identification.  A controversy surrounding the Bright Start college savings fund has been dogging him for months, but may soon reach a resolution.

Cheryle Jackson: The head of the Chicago Urban League and a former Blagojevich spokesperson, she announced last month that she was forming an exploratory committee for the race.  If she's ultimately the only woman in the race, Jackson will be one to watch.  And as Greg Hinz wrote back in February, "[p]robably the last thing embattled U.S. Sen. Roland Burris needs now is for another prominent African-American to make a move on his Senate seat."

Chris Kennedy: After stirring up a good deal of buzz in the Illinois political media in the spring, Kennedy has been laying low as of late (most likely waiting for Madigan to make her intentions known).  Little is known about the Merchandise Mart Properties president's political views or his reasons for running.  A poll cited by Lynn Sweet in May showed him with low name identification and indicated that his Kennedy roots neither help or hurt him in Illinois. A controversial business deal he orchestrated in Cleveland could become a campaign issue if he jumps in the race.

UPDATE III (2:50 p.m.): Giannoulias responds to the news:

"In my experiences working with Lisa Madigan and the Attorney General’s office, I know her to be a talented, dedicated, and principled public servant who has always put the interests of the state first. Her decision to seek re-election ensures that the people of Illinois will have a strong advocate on their side.  Our offices have worked very well together in the past and I look forward to continuing that relationship as we both focus on helping Illinois families get through these extremely difficult economic times.”