As Caroline Skinner revealed at the official Doctor Who convention in Cardiff and Digitalspy.co.uk says, the shooting location for the fifth episode of seventh series of Doctor Who will happen in New York.
There is a 2 page interview on IGN tv about the reasons of this departure: she needs a change. It was her decision and she talked to Steven Moffat about leaving the show. How she'll do it is still a secret, but it seems that she's leaving in the blaze of glori.
As far as I read on the internet, it seems that we have to wait about 9 months for the next episodes of Doctor Who. They are meant to be released only in the autumn of 2012.