There seems to be such a line. And like the real Mason-Dixon Line, it's not marked so clearly that the typical American can see exactly where it is. But it's definitely there.
Somewhere between Washington, D.C., and Richmond, Va., servers in restaurants and diners start asking you "sweetened or unsweetened?" when you order an iced tea.
Sure, there are northerners who drink sweet tea, and there are southerners who don't. However, by and large, my observation is that southerners are much more likely to drink their iced tea sweetened than northerners. I have eaten at a restaurant in North Carolina where the default was sweet tea -- you had to request unsweetened tea or that's what you got.
I drink my iced tea unsweetened, although the thought of sweet tea doesn't bother me. But it's a little annoying that all of the iced tea beverages that you find in vending machines are pre-sweetened.
Yet the North-South divide over iced tea is not restricted to the sweetened-or-unsweetened preference. Consider this story.
I once ordered an iced tea at a restaurant in Massachusetts during the month of March. The waitress responded tersely: "It's not in season." Huh? I was flabbergasted. Peaches are not in season, but iced tea is always in season. If you want to make it, you just make it -- regardless of whether it's May or November. I thought to myself: How hard is it to get some tea leaves or tea bags and immerse them in hot water?
Apparently, iced tea is a seasonal beverage in New England. Not wanting to create too much of a fuss, I bit my tongue and settled for a cup of hot tea.
Earlier this month, the Roanoke Times newspaper (that's in S.W. Virginia) posted this survey on iced tea-drinking habits. It's a short survey so go for it.
This web page purports to give a "history" of iced tea drinking in America, but I can't vouch for its accuracy.