I agree with the "inversely proportional" idea, to a point. The NT brain is specialized to socialize: They are hard-wired to learn language, to recognize faces, the judge facial expression, and to recognize the human figure. However, they aren't hard-wired to do mathematics, or to learn to read, or to solve puzzles. The hardware's there; but the software has to be built up through practice.
Now take someone who isn't as good at those IQ-test type tasks as most people. Let's say he scores an 80 on an IQ test. He's not all that good at math or reading or puzzle-solving... but his brain is still hard-wired, like most NT brains are, to learn to socialize. And that means he will specialize in socializing, unless he is forced to or greatly desires to specialize in something else, because socializing has by default become his strength. If his IQ were average--if he were as good at the abstract tasks as most people--he might be more of a generalist; but with an IQ of 80, he gravitates towards his strengths and learns more than most people... ending up with a "social IQ" of maybe 120.
That effect only holds true up to a certain point, of course, because there are abstract-reasoning aspects to socializing--remembering names, understanding pop-culture references, having a serviceable vocabulary. But still, in people who are across-the-board delayed in abstract, logical, IQ test type tasks, the socializing "wiring" of the brain is not necessarily delayed by nearly as much... resulting in things like a charming Down Syndrome child with social skills hardly delayed at all. (I do think this is where some of the acceptance of people with mental retardation has lately come from--that they have social skills enough to communicate to other people that they are undeniably human. The other part is of course the impulse to protect a child who is more vulnerable than other children, and it doesn't apply as much to adults.)
Are people with very high IQs socially delayed even without autism? They can be, but only through lack of practice. A very high IQ isolates a child from his peers because he is interested in more complex things than they are interested in. And a high IQ can be seen as intimidating. Many gifted children hide their giftedness just for that reason--they want to be accepted, seen as normal. However, many gifted people are also highly social; these people often make good leaders because they are not just socially perceptive but have a good grasp of problem-solving as well.