Certainly he has some ties to sketchy people (though this is true of many people in politics), but I've seen no evidence that he's actually corrupt in a way that even approaches many Illinois politicians such as Blagojevich.
Not corrupt? He made the 10 most corrupt politicians for 2007!
“Senator Barack Obama (D-IL): A “Dishonorable Mention” last year, Senator Obama moves onto the “ten most wanted” list in 2007. In 2006, it was discovered that Obama was involved in a suspicious real estate deal with an indicted political fundraiser, Antoin “Tony” Rezko. In 2007, more reports surfaced of deeper and suspicious business and political connections It was reported that just two months after he joined the Senate, Obama purchased $50,000 worth of stock in speculative companies whose major investors were his biggest campaign contributors. One of the companies was a biotech concern that benefited from legislation Obama pushed just two weeks after the senator purchased $5,000 of the company’s shares. Obama was also nabbed conducting campaign business in his Senate office, a violation of federal law.”