Assuming you connect at ATL to return to FLL, you will have to clear immigration, claim your luggage, clear customs, and drop off your luggage with the TSA for re-screening. As I recall, the drop off point is immediately after exiting customs.
I recently did it at ATL and it was pretty easy. The only issues will be wiats at immigration and maybe customs. If you have an 8 hour layover at ATL, I don't think that will be much of an issue.