Deja vu.
That is the key. While Obama may not have been offering anything to Blagojevich, it is clear that Blagojevich was demanding some kind of bribe for the Senate seat nomination. If Obama knew that Blagojevich was trying to sell the seat for an ambassadorship or some other kind of presidential appointment and he didn't immediately notify the FBI, then he could be in a lot of hot water.
I don't see how Obama could not know that Blagojevich was trying to sell the seat. Blagojevich's own statement make it clear that Obama wasn't coming up with the right amount of cash incentive to get his choice nominated. Somebody was working with Blagojevich's aides to try to get some kind of deal in place. Obama may not have been buying, but he was clearly being solicited.