I got a new bicycle a couple of weeks ago (it is awesome), and today I went down to the police station to get a bicycle license. A license is not required, but I want to avoid a situation similar to what my friend encountered with his bicycle.
We were playing Ultimate in the park, when two women wandered by. They stopped at the tree near our bikes and bags, and started intently scrutinizing my friend's bicycle. Then one of them started yelling, "Hey, where'd you get this bicycle? This is my bicycle that someone stole last week. I swear this is my bicycle."
My friend denied that it was her bicycle and told them when and where he had bought it, but they were very adamant that they take "her" bicycle. They would not give up. He then pulled out the piece of paper he carried with him that was the receipt for his license. It indicated the license number and serial number of his bicycle. They had no choice but to believe him.
I doubt that something like this will happen to me, but if it does, I'll be ready. And my bike will be more easily returned to me if it does get stolen.