I'm not entirely convinced that there is too much social mobility in the US, however much that we are told that there is. This little graphic from the NY Times has some interesting data on the topic.
Clearly there is mobility within class/economic structure, however the way I interpret what people think about "class mobility" within the US is that anyone can get from the bottom of the economic ladder to the top. Unfortunately, only 2.5% of the bottom quintile made it into the top quintile over a 10 year peroid, while 52.5% of the top quintile stayed there.
I appreciate that the folks in the top quintile aren't exactly rushing out to give up their spot (I'm certainly not), but I think the average American thinks its much easier than 2.5%. The graphic has a tab with "A Nationwide Poll" with some data that shows Americans think its alot easier to get to the top than it actually is.