It turns out that the offices of a Bill Ayers-founded organization (the Small Schools Workshop, "SSW") and that of the Chicago Annenberg Challenge ("CAC") - headed by Barack Obama - were in the same office building between 1995 and 1998. And, their offices were on the same floor of that building during at least part of that period. The CAC gave the SSW over $1 million and other foundations gave them almost another million during BHO's tenure at those organizations.

Details on both here.

The building in question has recently been demolished (link), but it was located at 115 S. Sangamon Street, Chicago, Illinois 60607. That appears to have been a University of Illinois at Chicago (UIC) property called the "Sangamon Building" (link) or the "Sangamon Street Building" (link) and having four or so floors.

The SSW was a UIC effort (link) headed up by Mike Klonsky; per this he was leader of the Marxist-Leninist Communist Party, which was effectively recognized by China as the all-but-official U.S. Maoist party.

In what appears to be a small office building it would certainly be difficult for Obama, Klonsky, and perhaps Ayers not to have frequent contact, especially since those offices were on the same floor. However, the next round of explanations from the Obama campaign is eagerly awaited.

UPDATE: There might be photos of 115 South here. However, the PDF here identifies what looks like the same building as 125 South. Those could be connecting buildings, a "115-125"-style address, or a mistake.