Is it the top foundry? It depends on what sectors you look at, I believe. It's definitely still top in web technology. But in other areas I'm less sure.

One slice of the successful startup market is looking at tech IPOs, which are on the upswing again. This probably downplays the Valley scene, which focuses more on quick smallish exits (acquihires and other kinds of acquisitions), but does give a view of where the next-gen of large tech companies is coming from.

According to PWC's Q2 2013 tech IPO report [1], there were 13 North American tech IPOs:

   EVERTEC: $505m, Puerto Rico
   CDW: $395m, Chicagoland
   Tableau Software: $254m, Seattle
   Blackhawk Network: $230m, SF Bay Area
   Gogo: $187m, Chicagoland
   Gigamon: $128m, SF Bay Area
   Cyan: $88m, SF Bay Area
   Rally Software: $84m, Boulder CO
   ChannelAdvisor: $81m, Research Triangle NC
   Marketo: $79m, SF Bay Area
   Tremor Video: $75m, NYC
   Textura: $75m, Chicagoland
   Halogen Software: $55m, Ottawa Canada

The SF Bay Area is still the largest single source, with 4/13, but not in a dominating sort of way.

[1] http://www.pwc.com/gx/en/technology/publications/assets/ipo-...

edit: Oops, those are millions, not billions; fixed. Nobody is raising a half-trillion dollars in an IPO...