I just got a call from an employer who had my CRB (criminal background check) form returned because I failed to put down my previous surname. Yes, I was as confused as you are. Apparently Ms means to some people widowed or divorced.
If I had been in the room by myself when I took the call I might have thought that it was just me being ignorant about the actual meaning of the term Ms. Fortunately Jessamy, who knows everything, was sitting next to me and she was as amazed as I was. We decided that it was probably just another example, like the term "you all right," where the British and American language differ.
So I looked it up online (how I love online), and found an article about the term Ms from The Guardian, a UK newspaper. The guardian seems to agree with me, that the term means I decline to state my marital status.
Since both the woman who called me and the CRB people who returned the form both believed it meant widowed or divorced I looked some more, and saw the question "Meaning of Ms, Mrs, Miss?" on
yahoo answers. Most seemed to think that it mean "declined to state" but there were a few who said widowed or divorced.
Oh well, seems like I'll either have to ask everyone who wants to know what their personal interpretation of Ms. is or write in longhand across all the tick boxes the words "None of your business". Sadly I'm not that antagonistic and I'll just go with Miss from now on.