Off tangential interest, filming on Sherlock Series 3 has been pushed to March 2013. I'm not surprised by this. Reshoots on The Hobbit were scheduled for after the New Year, and a two month delay makes Martin Freeman available for them. Plus, a two month delay in filming gives Steven Moffat some downtime between the end of filming on Doctor Who and the start of Sherlock. My guess is that The Hobbit was the main factor in the decision, though I'm sure Moffat isn't complaining. Will this have an effect on Doctor Who filming in 2013? Perhaps.
I think the 2013 shoot blocks have been agreed upon in prinicple already, since they apparently have plans on everything they need to do leading up to the anniversary in November. Assuming they want to do something DW for Christmas next year, they'll have to do it around the same time in the summer as usual, so it'll be pretty solid already. Moffatt will be glad to have a little extra time to polish his Sherlock scripts, but I'm sure he'll be just as hands-off during filming as he has been on his two shows so far. Mark
With the way Freeman and Cumberbatch's careers are going, I'm glad we're getting a third series, period.