As far as my unofficial survey of libraries using WiFi, I only got a few responses. Most libraries said they disable their WiFi when the building itself isn't open, citing security and bandwidth concerns. A few just leave their access point open and said they don't care who is using it. A few have patron-only authentication. Some are part of campus-wide systems where the library is one of many nodes and they do nothing particular as "the library" as opposed to part of the WiFi network. It's a whole new paradigm. Along those same lines, here's a column about things to consider when securing WiFI in a public library