As anyone here will readily tell you, the MA Bay Area is undoubtedly an awesome place to be right now in the sphere of cleantech. During the last couple months, I've been trying to put a finger on what exactly makes this place so special. Besides the high density of players in the New England energy community (Dave Danielson's point), an exciting piece of this puzzle is growth potential.
When one compares the distributions of population and solar energy companies throughout the U.S., one can't help but notice the high population density east of the Mississippi. The strong industrial, technology, and academic traditions of the Bos-Wash and Chi-Pitts corridors creates unique and favorable conditions for new entrants. Don't be surprised to see East Coast brands (e.g., IBM) as well as new entrants (spin-offs) joining in during the next months.
(this map is by no means complete!!)
Nocturnal view of U.S. from space, from GE Energy
Solar resource base of Germany and the U.S., compared on same scale. Source: SEIA