STYLETHREAD -- LET'S TALK SHOP!

I would interpret this as pretty dressy, but not necessarily black tie, and I'd err on the side of being too dressy as opposed to not dressy enough. I'm not from the South but my guess is that this would end up being a traditional wedding with people dressed to the nines. For the church, definitely cover your shoulders. Like, a cute little jacket/bolero over your dress or a wrap to be removed later.

I agree with Scarlett -- err on the side of formal/traditional and cover up for the mass.  I was at a similar wedding recently and most women wore cocktail dresses with wraps/boleros for the mass and then removed them for the dancing at the reception (strapless was pretty popular).  The reception at a plantation house definitely sounds more formal than not.  What were you thinking of wearing?  Are you looking to get something new or make something you have work for the occasion?