The people who were likely to vote Capuano in Somerville are not the "latte liberals" who are mostly newcomers, but the old-school entrenched Somerville residents from it's Slummerville days.  Just from my personal experience with the people I know in the area.  Most of the "latte liberals" I know in Somerville didn't bother to vote this time.  And I know a whole lot of them.  But the ones who have lived there forever sure did.

Boston may not care what Somerville thinks, but the old-school Boston is still more likely to go for someone of Capuano's personality, although again, these are not necessarily the same people who came out en masse for Obama.  I doubt the younger voters and minority voters really gave a cr@p about Capuano, and most of them probably didn't come out at all.  But there is still the entrenched political Irish who would be more likely to support Capuano.

Again, I realize they are the same areas, but there are a number of different demographic groups within these areas and they don't break down the same way.  

Obama won a LOT more of the state than Capuano did, although yes, some of those areas did overlap.  Coakley cleaned Capuano's clock in the westernmost part of the state that Obama won, and Capuano didn't win any areas inside 128 outside of Boston, Somerville, Cambridge and Chelsea.  While these neighborhoods may contain some of the upper-middle class vote, they are also more urban areas and contain a lot of lower income folk as well.  They are hardly Newton's or Weston's or Lincoln's that Obama won and Capuano lost.