My wife had a 40 minute connection at ATL last week. She was also connecting there from MCO. The MCO-ATL flight is very short and they seem to regularly land 10-30 minutes earlier than the scheduled time, when there are no weather or operational issues. She was also nervous about this though. Her flight from MCO landed 20 minute early and she had time to transfer from the B to A concourses, and even make a stop in the lounge for about 15 minutes.

If your first flight is on time, or close to it, you should be able to make a connection between any two concourses at ATL in that time.