Social mobility is a different story. I was raised in a rural area, which is essentially a social class of its own. Since becoming an adult, I've found myself gravitating towards working-class people. I like the bluntness, the awareness of real issues, and the family values. I make more money than many of the yuppies who live a mile away, but socially, I'm several rungs "below" them.
I could get a nice haircut, mow my lawn properly, get divorced, and fit right in, if I chose to, but I don't want to. I think you'll find many lower-class Americans wouldn't, even if they knew how to.
Edit: by "Working class", I mean blue-collar and not wealthy. The salary cap is a little higher than the traditional definition, but I think it's a more accurate representation of middle America.