There's an excellent graphic available on nytimes.com showing that median income/price relationship, for a variety of US metro areas. Think "three" (or thereabouts), absent a source of distortion (e.g. vacation communities).
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As for how - seemingly- so many could afford those pricey homes - well, I think that's playing itself out pretty clearly right now - "they can't." Not much different from: how can investors seemingly buy stocks selling at 50X earnings and still make money?
A source of information I find very useful on this income/price question is the American Factfinder:
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can look up detailed statistics about households and their incomes, for specific counties and many cities/towns. I use this to dispel clients' notions that "all my neighbors make over $200k." The percentage is usually small even in affluent areas. And 3X $200k is $600k (see above).
-Tad