What year is your car? That does seem pretty bad for something <10 years old. Only car I've seen w/ similar sag was an 80's BMW that lived most of its life parked in direct sun.
I wonder if Atlanta humidity contributed to that.
Anyways, you could try pinning it up. Haven't done this myself, but I'd locate some long, narrow pins, coat them w/ super glue (or similar adhesive) and stick them in while stretching the headliner out.
Do have a sunroof unfortunately. Yeah i do want to eventually just replace the whole headliner, but it's not the most urgent project. If you do ever have some free time, would love to have some help on this venture.Replacing the whole headliner is not bad, I've done a few cars. You can buy a fairly close match fabric at JoAnn's for ~$30. The headliner adhesive is $17 at Autozone.
Do you have a sunroof? Removing the sunroof isn't bad, but if you don't have a sunroof, it makes it that much easier.
There's a good write up in the stickies. If I can find the spare time, I'd be more than willing to help out and walk you through it.