i've generally found delta's BOS-ATL prices to be the same as airtran's all the way up to zero days. they're capacity controlled, of course.
Airtran's lowest fares tend to be heavily capacity controlled, and as a result, it's often cheaper to fly delta on 2-3 weeks advance. i suspect delta is very much looking forward to using T class to match airtran's lowest fares on nonstop routes.....
Edited to add: my experience with delta having better availability at low fares seems to agree with the delta advertising referenced by the original poster.
[This message has been edited by Seat1A (edited 07-11-2002).]