It is two days before the flight.

As posted earlier it was extended to 2 days for Elite members.

EDIT:

I fly Airtran frequently and recently have been monitoring seat maps and bookings. Only once or twice during very high travel days has Business Class been full two days out.

I still get complimentary upgrades well over half the time.

I also see many unassigned seats in the main cabin up to several hours before flight time, indicating that many passengers don't bother to check in on line or until they get to the airport.