Doctor Who Magazine
In DWM Steven Moffat has his own page at the beginning of the magazine called "Production Notes" well in DWM 426 (That's the Cybermen Special and it's why I own it!) Steven goes into explaining the series split and there are a few thing which come to the fore.
1.) Dear Steven even isn't sure even at this time, as to how the series will be numbered in the future, seeing as he's called the series 1, 5, 11, 31 and Fnarg by his own admittance. Fnarg being my personal favourite.
2.) Spoilers or not, so if you don't like possible spoilers skip the first paragraph that Steven has written.