I'm suspicious of the claim that there can be more than one silicon valley. With founders being more mobile and VC in... shrinkage (less willing to take risks setting up in new markets), the likelihood of any such concerted effort being possible is nil. After all, you're not going to get anyone outside the organizers to move from the valley in significant numbers, so I think the prospects for natural growth is limited.

I suppose what you say is true, but I don't see any reason to think such a transplant would ever outstrip the size/clout of the valley. And that's what it comes down to, is that with the big players already entrenched, and the weather so nice, no city will come close, even if the transplants do prove a boon.