From CNN:

White House Spokesman Robert Gibbs confirms that President Barack Obama will travel to Copenhagen, Denmark to make a big push for his home town of Chicago to host the 2016 summer Olympic Games.
Obama will join First Lady Michelle Obama and other administration officials in pitching Chicago to the International Olympic Committee on October 2.
No other U.S. President has ever attended an IOC vote.
Chicago is vying for the Summer Games against Madrid, Tokyo, and Rio de Janeiro. Leaders from Brazil, Spain and Japan are expected in Copenhagen to also make an in person pitch.
The U.S. has hosted four other Summer Olympic Games, 1904 in St. Louis, 1932 and 1984 Los Angeles, and most recently Atlanta in 1996.