Elizabeth Kolbert has an interesting review essay on recent happiness research - an emerging subfield of various social sciences - in the March 22, 2010 issue of The New Yorker (here). Using mostly survey techniques, researchers have gotten some intuitive and some counter-intuitive results. As we would expect, wealthier people within a given country tend to be happier than poorer people. More surprisingly, the percentage of Nigerians who consider themselves happy is about equal to the percentage of happy Japanese, even though Japan's Gross Domestic Product is nearly 25-times higher than Nigeria's. Based on such findings, some have hypothesized that happiness is relative to some changeable and usually local baseline.
If happiness is, indeed, relative to some changeable baseline, then I'm not sure that self-reported happiness or unhappiness is a particularly useful guide to public policy. Happiness research might yield some useful information about the declining marginal utility of consumption, a phenomenon Jeremy Bentham pointed out more than two centuries ago. For example, the installation of indoor plumbing is likely to improve people's lives, both objectively (material welfare) and subjectively (happiness), a lot more than a subsequent installation of gold-plated faucets. Beyond that, it's pretty clear that happiness, while an important goal, is not the only goal for most people. That is to say, happiness does not equal preference satisfaction.