The traditional claims for success/failure of companies inside/outside of the valley comes from Venture Capitalists who have years worth of statistics coming from their own investments.

The stats point to a change in percentage chance of success for any incremental distance from Palo Alto/Mountain View that a venture funded company is moved. (I don't know the numbers offhand - but it's significant, even down to whole percent numbers per 10 miles if I remember correctly)

From someone who's recently been talking to many people about the choice to live in Boston, Mountain View/Palo Alto, San Fran - or somewhere else, every person (investor/entrepreneur/historian) points to MV/PA.

Many people (and firms like Sequoia ...etc) won't invest unless you're in the valley.