Producer Sue Vertue tweeted this pic of John and Sherlock as they'll appear in the special, which is set to premiere next Christmas before season 4 the following year. So close, yet so far.
Speaking to Telegraph, Martin Freeman said filming a special for Sherlock in 2015 looks pretty likely:
"If that's going to be a special - I'm speaking off-message here; if this was New Labour I'd get fired - I think that might be for next Christmas. A Christmas special. That's what I understand."
UPDATE: Steven Moffat has commented and said there will be "a special, plus a new series of three episodes — it's a record-breaking run! Of course, it's far too early to say what's coming, but we're reasonably confident that the very next thing to happen to Sherlock and John, is the very last thing you'd expect…"