The only cultural thread I've been able to pick up that is common to all the U.S. tech hubs is great baseball teams.
I kind of want to plug Austin, though, as the third tech hub. There have already been some successful companies come out of Austin and Capital Factory is trying to build a YC of Austin of sorts. With Austin being a top-5 fastest growing US city over the last 10 years[1], plus the awesome climate, relatively reasonable cost of living and UT's engineering program, I think the pieces are in place for some exciting stuff to happen here.
[1]http://money.cnn.com/galleries/2012/real_estate/1204/gallery...
Austin is a great city -- they need to figure out how to get successful people to stay there.