Mark Perry has some interesting statistics about income mobility. I suggest you read his entire post here.
MP:A common misperception is that the top or bottom income quintiles, or the top or bottom X% by income, are static, closed, private clubs with very little turnover – once you get into a top or bottom quintile, or a certain income percent, you stay there for life, making it difficult for people to move to a different group. But reality is very different – people move up and down the income quintiles and percentage groups throughout their careers and lives. The top or bottom 1/5/10%, just like the top or bottom quintiles, are never the same people from year to year, there is constant turnover as we move up and down the quintiles.