The Politburo no doubt had internal investigations, and so too does the incoming Barack Obama administration. They've just released a five-page PDF completely clearing everyone of everything; cached copy here. It includes the news that Obama was interviewed as part of the investigation:

These accounts were communicated to the Office of the United States Attorney in interviews that were conducted last week. At the request of the Office, we delayed the release of this report until such time as the interviews could be completed. The interviews took place over a period of three days: Thursday, December 18, 2008 (the President-Elect); December 19, 2008 (Valerie Jarrett); and December 20, 2008 (Rahm Emanuel).

Hugh Hewitt thinks the order of questioning doesn't seem right.

Per the report, Obama "had no contact or communication with Governor Blagojevich or members of his staff about the Senate seat". However, BHO used David Axelrod and Rahm Emanuel to communicate a list of "qualified candidates" to Rod Blagojevich after Valerie Jarrett pulled out of the running. Those on the list "included" (were there other names?): Jan Schakowsky, Jesse Jackson Jr, Dan Hynes, and Tammy Duckworth:

Mr. Emanuel subsequently confirmed to the President [sic] that he had in fact relayed these names. At no time in the discussion of the Senate seat or of possible replacements did the President-Elect hear of a suggestion that the Governor expected a personal benefit in return for making this appointment to the Senate.

As for Emanuel, soon after he accepted the job as Chief of Staff he called Blago and:

They spoke about Mr. Emanuel’s House seat, when he would be resigning and potential candidates to replace him. He also had a brief discussion with the Governor about the Senate seat and the merits of various people whom the Governor might consider. Mr. Emanuel and the Governor did not discuss a cabinet position, 501c(4), a private sector position for the Governor or any other personal benefit for the Governor.

Per the report, Emanuel had four conversations with John Harris, Blago's Chief of Staff. He added other names to the list: Lisa Madigan and Cheryle Jackson. Per the report, Harris didn't bring up any quid pro quo.

However, a qpq was mentioned two other times (per the report):

Ms. Jarrett recalls that (Tom Balanoff, the head of the Illinois chapter of the Service Employees International Union) also told her that the Governor had raised with him the question of whether the Governor might be considered as a possible candidate to head up the Department of Health and Human Services in the new administration. Mr. Balanoff told Ms. Jarrett that he told the Governor that it would never happen. Jarrett concurred.

And, Illinois Deputy Governor Louanne Peters called Obama friend Dr. Eric Whitaker wanting to know "who, if anyone, had the authority to speak for the President-Elect". Then:

The President-Elect told Dr. Whitaker that no one was authorized to speak for him on the matter. The President-Elect said that he had no interest in dictating the result of the selection process, and he would not do so, either directly or indirectly through staff or others. Dr. Whitaker relayed that information to Deputy Governor Peters.

Everything's on the up and up.