While there is no indication whatsoever that Attorney General Lisa Madigan is even considering such a move, Mick Dumke planted the seed in a blog post yesterday:

I should also take note of another seat up for election just a few months after these, in early 2011, that none of these politicians, or any others of prominence, has openly discussed so far. The incumbent has been taking some major political hits lately. It's the office of mayor of Chicago.

Lisa Madigan could alter the future of the city just by running for it and asking a few questions about, say, hiring policies, government transparency, financial management, and privatization deals. Will she? Would she?

Doubtful.

Doubtful, indeed.  But it's a fascinating scenario to contemplate.  While the Daley and Madigan clans have co-existed in Illinois for decades, there has also been a simmering rivalry between the House speaker and the mayor.  The Tribune's Rick Pearson recounted some of the backstory in a 1997 Illinois Issues article:

[Mike Madigan] was able to adapt and survive the revolutionary changes in city politics that stretched from Harold Washington's administration to that of the current mayor, his former legislative colleague Richard M. Daley.

Relations with the younger Daley had changed, however. In 1980, Madigan joined with many of the late mayor's supporters and endorsed the candidacy of 14th Ward Aid. Edward Burke over Daley's son for Cook County state's attorney. It's something the second Mayor Daley has never forgotten. [...]

[Madigan has] never been afraid to take on the mayor. In 1993, when Daley said a Republican-backed property tax cap in Cook County should be considered as a trade for money to keep city schools open, Madigan fired back. He advanced a Republican bill to kill a $6 million soft drink tax Daley was counting on to close a hole in the city budget. 

Fran Spielman also delved into the relationship in a January 2002 Sun-Times article:

For nearly 13 years, Daley and Madigan have pushed Chicago's legislative agenda with an arm's-length suspicion fit for a husband and wife who each fear the other is cheating. [...]

[T]he rivalry has stumbled into campaign after campaign. In 1994, Daley backed John Stroger for Cook County Board president and provided behind-the-scenes assistance to Democratic gubernatorial candidate Dawn Clark Netsch. Madigan supported Aurelia Pucinski against Stroger and County Board President Richard Phelan for governor.

In Springfield, Madigan has been supportive of many of Daley's mega-projects, but he has also been quick to flex his legislative muscle whenever Daley says or does anything to undercut his position.

Speaking of "campaign after campaign," it's worth remembering that Daley endorsed Lisa Madigan in the 2002 attorney general primary -- over his former chief of staff, no less. It was quite unexpected at the time and not something the Madigan clan will soon forget.