I chatted a moment with the rider for just a moment, and it's a project of the city of Cambridge to collect the recycling from bins on the street using bikes as the collection vehicle. A lot of the dense city squares (Harvard, Porter, Central) have on street recycling cans, and no parking, so this is probably a pretty efficient way to empty them. It's a Metro-Pedal Power bike. It seems like they've gotten to be a pretty big business here- I'd love to go see their facility sometime and see what all they're up to.
I like the wheels, although I couldn't quite make out what they said.