I like open source as much as the next guy, but I don't know that I agree - is open source the cause of concentrations of 'interesting' things, or is it the other way around? Silicon valley wasn't built on open source....
Besides, plenty of hackers do interesting stuff outside outside the open source world (and that's not even considering hardware...).
I think open source has very little to do with being a 'cause' for concentration. What's interesting about it is it shows, to some degree, the natural distribution of high quality hackers throughout the world. And yes, of course people do interesting things outside of OS, I was just suggesting that it's a good proxy to see some areas where innovation is happening that don't have some of the other ingredients of SV.
Besides, plenty of hackers do interesting stuff outside outside the open source world (and that's not even considering hardware...).