Last year in an article referenced by Fierce Wireless, I read that the carriers were going to start adding a "feature" like this in order to try to reduce data load on their networks (of course, it was to be pitched to customers as a feature to speed up their network access). In effect, any time your phone was near an access point to which it could connect, it would automatically turn on WiFi. Around February, 2012, this started happening with my Samsung Epic 4G, right after a software update from Sprint. It is a real pain, particularly because it drains your battery. As an example, if you have a home AP, and you sometimes connect to it from your phone but not always, you will find that every time you are at home your phone is constantly turning on WiFi and draining the battery. I thought it might be just my Samsung, but two weeks ago I got a new Motorola Photon Q LTE. It is doing the exact same thing, although it does have the courtesy to pop up a message box each time it does this, and ask me to "make this setting permanent". I have discovered that if you do not elect to make the setting permanent, it acts as if you did select that anyway.
I have not yet found a way to disable this "feature", but I would really appreciate if anyone else could find an effective and simple way. So far the only halfway effective suggestion I have seen is to "forget" all of your WiFi connections. Of course, if you occasionally do want to connect to a secured WiFi AP, then you will have to reenter the security key each time, which sucks. Or you could just decide you will never use a secured WiFi AP again, which also sucks.