H.H. in Ramsdale

Humbert Humbert's reasons for staying in a sleepy New England town, as he says in the text revolve around a slowing of pace--"I cast around for some place in the New England countryside or sleepy small town (elms, white church) where I could spend a studious summer subsisting on a compact boxful of notes I had accumulated and bathing in some nearby lake" (35). This makes some sense, as he thinks the outdoors "promise [him] some reflief"--he goes on an expedition with a doctor-friend before checking out of the sanatorium and finding his way to Ramsdale, and a "sleepy New England town" promises some sort of outdoors-y attraction and activities (33). However, as soon as he learns there is a twelve year old girl who resides at the McCoo's, the family he is to stay with, his mind largely fleets from thoughts of rural relaxation and is set on secretly "fondling her in Humbertish" (35).
So though Humbert's primary reason of picking a sleepy rural place to reside is his sudden desire to return to his studious ways, Nabokov's may be a bit different. I think there is some relation to this choosing of the setting to the sorts of people who will reside there--"middle class [Americans] in the '50s," as you say, Robin. Sleepy New England is precisely the place to find single mothers with wild-child daughters; a place where many women are, as Humbert describes, "those women whose polished words may reflect a book club or a bridge club, or any other deadly conventionality"--the northeast in the 1950's was surely a hot-bed for convention. Nabokov chose Ramsdale as the setting because small towns are so set on their own history and convention--placing a man like Humbert in the middle of small town American tradition may not seem like deep enough of a rift at first, but his arrogance and cynicism are something that directly conflict with the warmth the idea "American tradition" presents. The story's place in Ramsdale is also important because it is a place where a pedophile can lay low--a place where, as long H.H. keeps up appearances, there will be a miniscule amount of suspicion.