Apple opened up iOS 5 Beta 1 to developers earlier this summer, and since then, there have been 3 updates. Given the nature of human curiosity, you can guess that it hasn't just been developers taking the new firmware for a test drive.
There's a few ways for non-developers to acquire a copy of iOS 5, but the most popular method of late has been by purchasing a UDID slot from a person or business with an Apple developer account. It looks like those days may soon be over...