it's cube of office space that GM is going to move into, plus the adjoining tower which is what you see over the OF wall. The tower is supposed to be apartment/hotel or some such residential typ space. It's being put up by Gilbert/Bedrock. They've completed the structure now, but last night I was at the ballpark and you could see that the interior of the tower still had construction lights hanging inside - just open space yet. The kicker is that when GM moves there the RenCen ends up with a bunch of vacant space and they've floated a plan to tear down the two towers closest the the river to open up some green space there. Not exactly Orthanc and MinasTirith, but the complex would still look pretty weird with the three still standing in such an odd asymmetry.