http://www.snopes.com/rumors/flight.htm
SNOPES: Members of the Bin Laden family and several
Saudis MAY have left the US while almost all flights
were still grounded until Friday the 14th of
September, 2001. I have not found any evidence
otherwise. Not in any Newsweek article or in the 911
commissions reports, but I did find this on the Snopes
Urban Legend Reference pages (link to follow):
"However, records obtained from the U.S. Department of
Homeland Security under the Freedom of Information Act
(FOIA) seem to indicate that one flight carrying
approximately 46 Saudi citizens may have left the U.S.
from New York as early as September 13, before the
general ban on air travel was lifted. The records do
not identify who these passengers may have been — bin
Laden relatives, royal family members, or other Saudi
nationals. (The "Class of Admission" column in the
document lists the departing passengers as a mixture
of foreign government officials and their employees
and temporary visitors to the U.S. for either business
or pleasure.)"