Ryan Murphy provides a link to these big business/U.S. government Venn diagrams.
I'm never quite sure what to make of these worries. I just try to think of what the world would be like if government officials didn't play important roles in the private sector in the fields that they deal with as policymakers. Is it possible to have a policymaker knowledgeable about the impact of his policy in the real world without inevitably having many of them rise to the upper ranks of business? Would we really rather have a situation where we deny people with this kind of experience any role in government? Cronyism to me is about a lot more than "six degrees of separation" test. I'm much more concern about whether there is actual malfeasance. That's obviously a risk - you don't need to be a public choice theorist to know that. But the risk on the other side is incompetence. When we have costs and benefits to weigh on either side, that usually means we have a trade off to make and it usually means a corner solution is not the right solution.