Winboni is an autonomous robot just five inches square and two inches thick; it runs on AA batteries and attaches itself to the window with a suction fan. Little powered wheels move it about and felt pads perform the cleaning actions.
Kyle Koepf, Jonathan Luckhardt, Emily Duszynski and Joshua Thomet tried five different designs and spent more than 900 hours in meeting the contest specifications.
As Robert Heinlein might say, there's some good skull-sweat in this 'bot. He wrote the spec for Window Willie, which used electrostatics to clean polished surfaces.
I got to thinking about dirty windows and that ring around the bathtub that is so hard to scrub, as you have to bend double to get at it. It turned Out that an electrostatic device could make dirt go spung! off any polished silica surface, window glass, bathtubs, toilet bowls-anything of that sort. That was Window Willie and it's a wonder that somebody hadn't thought of him sooner...
Update 13-Feb-2009: Take a look at this entry for the window cleaner robots from Arthur C. Clarke's 1972 novel Rendezvous With Rama. End update.