The first restaurant I ate in Atlanta a few years ago only served pepsi, which was a weird circumstance to say the least...
There's actually a lot, not the "50:50" ratio you might get elsewhere, but I run into it all the time at lunch. You ask for a coke, because in Georgia, everything that isn't sweet tea (eww) is Coke. Then the waiter/waitress asks (thank $deity) if Pepsi is ok. And when you say no, unlike in other states, they don't look at you funny, they just understand.
Luckily.... most places carry dr pepper/diet dr pepper despite which main cola brand their distributor carries.