Megan McCardle talks about the difficulties facing people from poor backgrounds in moving up the ladder.
Another thing I would add to this list would be geographic mobility. One of the major differences between people in the lower, middle, and upper classes is how much they travel. Often, you'll find that middle and upper class families are spread across the country. They are willing and able to travel, and this means they can take advantage of opportunities that don't exist if you don't move. If you're only looking for jobs in only one area, you're hurting you're chances.
Also, It's kinda obvious to say, but poor people often come from poor places. If you're poor and never move you wake up each morning in Lone Pines SD, or Detroit, or rural Alabama.
Partly, I think that this lack of mobility is due to the fact that upper and middle class people are not as dependent on local support networks, whether that is relatives or friends or the church. Also, they have the resources to move, as well as to still stay in touch with family: they can afford a cross-country plane ticket to come home for the holidays. Middle and upper class people also have those connections in places outside their local community: schoolmates (say from college), relatives, co-workers who can tell them about job opportunities that they wouldn't be aware of otherwise, help them settle in, and can make moving less of an alienating experience.
When you grow up in W. Baltimore, or Appalachia, and you and every one you know has never been anywhere else, that anywhere else can seem foreign in a way that the middle class and the upper class just don't see.
