Obama is reportedly choosing Valerie Jarrett as his chosen heir for his now-open Senate seat. Reviewing Mrs. Jarrett's history it would appear that the main thing on her resume that qualifies her to represent Illinois is the fact that she's a long-time friend of the Obama family .

http://politicalticker.blogs.cnn.com/2008/11/09/source-obama-wants-valerie-jarrett-to-replace-him-in-senate/

The decision of who succeeds Obama actually belongs to the governor of Illinois, Democrat Rod Blagojevich . With Governor Blagojevich's 4% approval rating in Illinois (the lowest approval rating of any current U.S. governor), presumably the governor will appoint whoever Obama wants.