A local news story had an interview with a Georgia World Congress Center administrator. It looks like the damage is being repaired and that the WCC isn't cancelling any events at this point. FLL is in late April, so I'm hoping that the damage will be taken care of well before then. I'm trying to get an official answer about FLL, but right now none of the websites (FLL or GCC) are saying that for 100% fact. I know a lot of people are concerned about air travel they've already arranged - I wish I could give you a firm answer, but right now all I can tell you is the news reports seems to indicate that the WCC is in operating order.
One thing you MIGHT want to do is double-check with your hotels - a lot of other buildings in downtown Atlanta were damaged by the tornadoes and I have no information on whether any are closed or have reduced room availability - do check that for yourselves.