After Nov 1 2010, airlines need to submit passenger details at least 72 hours before takeoff for the TSA to authorize (yes, authorize) the issuance of a boarding pass. They are allowed a specific percentage of "late" bookings for last minute purchases and such, but are discouraged from promoting last minute booking.
This is one of the reasons "last minute deals" emails you get from airlines are usually still 1 week+ out.