The city of Madison sweeps bike racks once or twice a year, and the abandoned bikes get sold off in the first week of May. I haven't found out when the University sweeps their racks, but I know they do.
Yep, this year they sold several almost-new Treks,a couple of nice Electra cruisers, several Gary Fisher MTBs, and some easily cleaned up vintage Schwinns...wish I had more space in my garage!