National Average Hourly Rate for Law Firms is $372
According to the National Law Journal, the average hourly rate for U.S. law firms rose 2.5% in 2009 to $372. The average rate is $457 for partners and $282 for associates.
Four law firms reported average partner billing rates of $1000 or more. The article quoted someone as saying that those rates typically came with specialized practices and close relationships with high-ranking government officials.
From viewing the list of firms that responded to the survey, it appears that the survey is reflective of average rates at the nation's largest firms. It does not appear to be a comprehensive survey that truly reflects the average rate among all attorneys in the U.S.
The highest rates in Mississippi that I am aware of are in the $500 per hour range. But most lawyers in the state charge significantly less.