I think PG and this group are too focused on Web 2.0 companies. So all his talk of superiority of the valley ecosystem is probably right in the consumer-web space. Boston stills kicks ass for enterprise software and biotech.Unfortunately you can't build those with college dropouts.
Once again, I would caution anyone who draws a line in the sand between Web 2.0 and the enterprise. It won't be too long before that line is blurred forever. (Aside: Ask Bill Gates what can be built by a college dropout.)
As for Bill Gates, that is an old example. If you look at even the last 15 years, the most successful enterprise companies (VMWare, Siebel, SalesForce, ...) were founded by seasoned, college-educated founders.