In fact, I would gander that there must be metropolitan areas of the world that have more startups per square mile/km.
Israel is 20,000km/sq
NYC is 1,200 km/sq (metro area 30,670 km/sq, but we can discount a lot of it if we're just looking for a concentration)
Boston metro area is 12,000km/sq
I'd imagine the "concentration" is higher in these two cities, but maybe not. Does anyone have any data?
EDIT: boston.areastartups.com claims that Boston has 1511 startups. This map lists 628.