I can't find a thing on AirTran's site, and would just call them.
On Northwest in their Special Luggage section:
Infant and Child Items
Northwest accepts certain infant and child items in addition to normal luggage allowances.
***
Strollers: One fully collapsible umbrella-style stroller may be carried onboard, in addition to the carry-on luggage allowance. Strollers are only allowed on board the aircraft if approved storage space is available.
Northwest also accepts child carrier backpacks and strollers as checked luggage. Each item may be checked in lieu of one piece of luggage included in the free luggage allowance. When these items are checked as luggage, all excess, oversized and overweight charges will apply.
OK well that's not helpful either, b/c it's either "carried onboard" or checked....frustrating!