Paraphrased version without spoilers: So the Word of God is that the 2015 Special is AU of Sherlock.
No, really. Only Mofftiss would make an alternate version of their own alternate version of Sherlock Holmes.
Steven Moffat has confirmed that the the #Sherlock special will be set in Victorian London. http://t.co/1vtxaJKx2L
— The Mary Sue (@TheMarySue) March 18, 2015
Much wondering in speculation-land has now been put to rest. :D
Check out io9's Christmas Special piece from last week along with EW's brief Moffat interview on 16-March-15.
UK air date is still said to be Christmas 2015. The PBS air date for USA is unknown, as are other countries's air dates for the special.