There is school choice in Cambridge, but I have a relative who works at Harvard and moved from Cambridge to Concord due to the schools. So my guess is that it's very limited with what Cambridge (or Somerville) public schools can offer at the elementary grades. There's a number of families who either send their gifted kids to private school or un/homeschool them or move out to the suburbs (ie. Lexington or Concord) as a result.
Cambridge Rindge and Latin may be an option for high school, but not sure what you do until then. There is a new Sudbury Valley type of school in Cambridge that recently opened, but no idea on the cost or other pertinent details (and can't remember the name at the moment either, grrr...). Sudbury Valley is located in Framingham and is probably too much of a hike for you.
Private schools in MA can be expensive and don't/won't necessarily accommodate hg/pg crowd based on my experience. Acera School now in Winchester (
http://aceraschool.org/) and The Birches in Lincoln (
http://www.birchesschool.org/) are newish and there's no guarantee that they'll work for you and your child.
