Sure, if companies start thinking about their supplier base just now, well that's just too late. I can only comment on physical supply chains, so. But even there building up suppliers takes time and money. Personally, I would love to see the risk of single sources be valued higher in the equations. One can dream, right?

Edit: Now I got the cluster thing. I meant cluster in the sense of risk, meaning you have a single point of failure and high degree of risk concentration. But your point is really interesting, I never thought about these aspects of cloud computing.