I can't guarantee this is the exact location of the ballroom scene. I did talk to an employee at the hotel who started working there in 1973 and she claims this is the location. Obviously, the space has been remodelled (probably a few times since filming). Not much to go on to verify this is the space.
I posted my location photos from What's Up, Doc? back in February 2010. But just when you think I've photographed every posssible scene, I have found a few more. I had already posted this location before, but I had the camera too high and the vanishing point was off. I've shot it again at a lower angle.