I found this very interesting article about the legal ownership of bees and swarms.
Just in case you ever have a dispute about who owns that swarm you just captured: you do, no matter who owned the parent hive or where the swarm was captured, even if you trespassed to capture it.
edit: I am not advocating you trespass to capture a swarm, and neither is the article. Always get permission, its the decent thing to do. I'm am speaking only on the ownership of the swarm. The article makes it quite clear to those that bothered to read it that a person may still have to answer the trespass. It is an item on the list of conditions that do not negate ownership of the swarm for the person who captured it, the others were ownership of the parent hive or where or when the swarm was captured.