Until recently it hadn't actually been officially confirmed anywhere, though the recent issue of DWM seems to confirm that the final two episodes of this series number as 11 and 12, are both written by Moffat and are both directed by the same woman
There was leaked documentation suggesting as such way back in 2013 though it fell under the radar as the same documents also contained information about Matt Smith's departure from the show. Out of nowhere, his departure was officially announced by the BBC the following day, which kind of lends credibility to the documents in question.
The filming schedule has also been significantly shorter for this series, despite the fact there was such a vast amount of time from last year that went unused. But they couldn't start filming with a new Doctor until he could be officially announced.
To be honest, 12 episodes will likely work out a lot more tidily than 13 does. 13 is a tried, tested and typical British series length, I think because it equates to exactly a quarter of a year for a show that airs once a week. It means the BBC are able to manage their budgets and their schedules quarterly, and it saves them a lot of time and effort to do so rather than having to commission lots of one-off dramas to fill the voids in between. It will also lighten the load on the Doctor Who cast and crew who often struggle around the 12/13 episode production mark - it's how we ended up getting Doctor-lite stories way back at Series 2, simply to alleviate the load from the actors.
But with the rise of on-demand viewing, other internet distractions and the fact that 'event TV' is still 'event TV' when viewed online, I think the 13-episode model is a bit outdated, especially for a show like Doctor Who. On demand has also meant that the Beeb can get away with filling the schedules with movies like Harry Potter and Shrek on Saturday evenings a lot, as people are less inclined to notice and less demanding of new content each and every week.
12 episodes for Doctor Who is fine. It's the most consecutive episodes we'll have had in over four years, and maybe the time and effort spared will mean the budget is better distributed and the writing is a little more tight. What will be interesting to see is whether the Christmas specials will now be a part of the series before it (as opposed to preceding the series that lies ahead), or whether like Series 1 and Series 5, we'll again have a series without a Christmas special.