Another stat, more important than the overall national debt, is the total
amount of debt held by the public.
It was 46.1% in 1997 and 33% in 2002.

From the CBO

"Businesses and the financial markets pay close attention to (publicly held
debt) it because of its impact on the economy. When the Department of the
Treasury sells securities--in effect, borrows from the public--that activity
absorbs resources that otherwise might be invested in the private sector"

http://www.cbo.gov/showdoc.cfm?index=3948&sequence=0