Are we talking for an individual, or for a married couple? For an individual, I'd say $25K - $75K is middle class. For a married couple...probably $40K-$100K.
And I disagree that it depends on where you live, or the number of people in the family (beyond the wage-earners themselves). Because people can choose to have 0 kids or 5 kids...and they can choose to live in rural Indiana or Manhattan. This is one of my pet peeves about some of my friends here in the DC area, who are fond of bitching that they'd be rich if they lived somewhere else in the country...well, they chose NOT to. I view it as akin to a guy with a Ferrari who complains that he's poorer than a guy with an equal income who has a Chevy Cavalier, because his car payments are higher.