Executive-producer Beryl Virtue has confirmed to the BBC's in-house magazine Ariel that Sherlock won't start filming its third series until "early 2013". Given the turnaround we've experienced with the previous two series, I predict this means Sherlock will be back in autumn 2013 (or perhaps late-summer, echoing series 1).
Sherlock unfortunately has a very long development process; mainly because co-creator Steven Moffat is so busy most of the year running Doctor Who, Martin Freeman has resumed shooting The Hobbit in New Zealand, and Benedict Cumberbatch's film career is about to skyrocket (he's currently playing the villain in JJ Abrams' Star Trek 2, for instance).
Thankfully, everyone involved is very keen to make more Sherlock, so this show isn't going away anytime soon. Fans will just have a probable 18-month wait for further adventures.