Well the Sherlock special has commenced filming and yes it would appear that Victorian costume is the order of the day.
The three scenarios doing the rounds are:
1) Some kind of dream sequence
2) An event at which Victorian costume has to be worn
3) A genuine Victorian set Sherlock to be held separate from its 21st century colleague
I think (1) is too much of a cliche as is (2) so I'm hoping for (3). I don't think this would do the main series any harm at all. Perhaps for some it might break the spell but I think for most of us it would be no problem at all.