A recent study of submitted tax returns (from 2007, since you asked – the most recent set of data available to researchers who study this type of thing) shows that IF YOU ARE MAKING OVER $32,878 A YEAR, then you are in the top 50% of income earners in the country.
$32,878 is the cutoff point, and the top 50% of all earners — those making more than $32,878 — cover about 88% of all income earned in the country.