At this point in time, the most significant problem we will face with tin whiskers, is the matte tin pure plating on the part leads.
Tin/silver eutectic may be capable of supressing the whisker growth, I know from experience that it absolutely stops plague, as all my experience has been either at liquid nitrogen or liquid helium temperatures. My applications required a 200 mil (2/10 of an inch) spacing gap between any joints, so what I've done is certainly not fine pitch.
Military use of lead free product will be an issue for several more years, as conformal coatings do not stop whiskering. Supplying lead free product in the military, medical, flight, control, space arena's is putting an awful lot of eggs in a basket of unknown quality.
Conversion to lead free is NOT without cost. Energy cost to make and use the solder, chemical cost in using non lead, engineering cost to design packages capable of the temperature increase, including parts molding as well as pcboard design rules.
It is not clear that environmentally, lead free is a net positive move for the environment. Shorter product life cycles because of whiskering cannot be a good thing.
Cheers, John
