How much water do you need to boil pasta?

A while back I realized that I was committing a cardinal sin - failing to put sufficient water in the pot when boiling pasta. Still, the pasta tasted fine to me, so I didn't change my ways. The New York Times seems to back me up:

SOME time ago, as I emptied a big pot of pasta water into the sink and waited for the fog to lift from my glasses, a simple question occurred to me. Why boil so much more water than pasta actually absorbs, only to pour it down the drain? Couldn’t we cook pasta just as well with much less water and energy? Another question quickly followed: if we could, what would the defenders of Italian tradition say? ... The standard method for cooking pasta, found in Italian cookbooks and on pasta packages, is to heat to a rolling boil 4 to 6 quarts of well-salted water per pound of pasta.

Interestingly, the article suggests that boiling pasta with less water might save $10-20 million in annual energy costs in the US.