Actually, there are many people in Twitter from Boston. Also the UK, Japan (so many that they've started Twitter Japan), New York, Kansas, Louisiana, and more. Twitter is the web. It's way beyond "California".
In fact, according to twitterlocal.net, there are roughly as many twitterers in New York City as San Francisco (on May 6 it was almost even). More in London UK than Los Angeles, CA. Almost as many in Wash. DC as LA. And Tokyo is the Twiteringest city by 3 to 1.