I don't think the definition of the official unemployment rate has changed any time recently. There are different definitions but the official rate is based on those actively seeking work. Here is a thorough article on the Wikipedia. (John B, cover your eyes!) It takes it all the way through U6, which seems pretty speculative to me. U3 may not be the perfect measure but at least it's based on real numbers.

http://en.wikipedia.org/wiki/Unemployment

>Remember that the unemployment rate is not "how many people don't have jobs?", but "how many people don't have jobs and are actively looking for them?" Let's say you've been looking fruitlessly for five months and realize you've exhausted every job listing in your area. Discouraged, you stop looking, at least for the moment. According to the government, you're no longer unemployed. Congratulations?
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>Since 2007, the percent of the population that either has a job or is actively looking for one has fallen from 62.7 percent to 58.5 percent. That's millions of workers leaving the workforce, and it's not because they've become sick or old or infirm. It's because they can't find a job, and so they've stopped trying. That's where Luce's calculation comes from. If 62.7 percent of the country was still counted as in the workforce, unemployment would be 11 percent. In that sense, the real unemployment rate -- the apples-to-apples unemployment rate -- is probably 11 percent. And the real un- and underemployed rate -- the so-called "U6" -- is near 20 percent.
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>There were some celebrations when the unemployment rate dropped last month. But much of that drop was people leaving the labor force. The surprising truth is that when the labor market really recovers, the unemployment rate will actually rise, albeit only temporarily, as discouraged workers start searching for jobs again.