Are postdoc salaries in the USA adjusted for cost of living?
There is a wide variety in the range of post-doc salaries. In the biomedical sciences, the NIH's salary guidelines hold sway, as the NIH is the largest source of funding for such positions. So there's relatively little cost-of-living adjustments. 

However, I think many institutes set their own scale according to "what the market will bear." Popular schools like MIT could pay somewhat less and still be competitive because of the "prestige factor."