Ok, I'm coming a little late to this discussion, but I'd like to add my two pesos:

I think it's inevitable that a single stack .45 in the M&P line will come out sometime in the next year or two. I don't see it being quite as small as the 9 or 40 obviously, but perhaps something with a grip about the length of the M&P45c but and with a shorter barrel. You could easily get 6 rounds in a gun that size.

I tried the XDs this past Saturday and let me tell you, I LOVED it. It's going to be extremely popular and S&W would be foolish to not try and cash on that market.