I'd actually extend the "startup corridor" out to include most of the Red Line, from South Station to Davis (and possibly to Alewife, soon). There're a bunch of financial startups in the financial district between Downtown Crossing and South Station - I interviewed at one and work at another. And the Porter/Davis area was the birthplace of a number of yCombinator startups, notably Reddit. Many of them are now in Silicon Valley, granted, but they tend to get started near Davis where rents are low.
Our startup is right in Downtown Crossing around the corner from the T entrance. We got a ridiculously good deal on rent. Once our lease expires we'll probably have to move, because there's no way it's getting renewed at the same rate.
I used to work for a company that entirely guided their office move by "is it still in Cambridge?". Gotta keep that 02138. Too bad they were an SBIR crackhouse.
But there is no denying the number of brains here. What did PG say? Silicon Valley has the VCs, Cambridge has the brains?