Frustrated travelers have banked enough Northwest Airlines bonus miles to crisscross the globe many times over but new restrictions have made it harder to cash in on free plane tickets.
In 2003, the airline said its frequent fliers had the miles to book 7.2 million round-trip tickets -- enough to replace the airline's paying customers for three months.
http://www.detnews.com/2005/business/0503/27/C01-130242.htm