He confirms the news on Twitter:
He akso tweeted that he'll be playing Henry in episode 2, The Hounds Of Baskerville, written by Msrk Gatiss and based around the most famous Arthur Conan Doyle Sherlock novel."Yep! I'm gonna be in BBC's Sherlock... Official."
Digital Spy reveals that Sherlock and Watson (Benedict Cumberbatch and Martin Freeman) are recuited by Henry after "suffering from a traumatic childhood experience."
So thus concludes my first Sherlock post!