From the point where I started paying attention, everyone said "vous" to everyone else, except Sherlock and John toward each other. In particular, I thought Moriarty might call Sherlock "tu" as a feature of his creepy-flirty and condescending manner, but no.
I wonder if Sherlock and John began using "tu" as soon as they became flatmates — it would be incredibly stiff to use "vous" over breakfast in a shared apartment with someone of roughly the same age.
Presumably, Mycroft and Sherlock use "tu" with each other as well, and I would certainly hope John and his girlfriends do. Irene and Sherlock... probably not, although Irene might switch to "tu" when she belittles Sherlock after tricking him. Hmm... kind of tempted to rewatch the series in French just for the pronouns.
edited 25th May '12 7:46:04 PM by Embryon
"If it ain't broke, don't fix it. And even if it is broke, just ignore it and maybe it'll be sort of OK — like the environment."