as for the stepping stone question: i have a friend who did a MSc at LSE (different subject area though) and then went on to do another Master's in the US. I guess it depends on what you have done before. But in general, as British MSc's are only one year and in the US, as far as I gather, normally Master's are two years, a British MSc won't automatically be recognized as equivalent to a US Master's.
On the other hand, if you already have a Master's equivalent, say from Germany, and really what you need is just the name of a top university, the stepping stone strategy might work. But then I'd ask why you don't try going for the PhD in the US directly.
Hope that helps.