If by nation you mean classical nation (from the root "to be born"), referring to a common identity ... kind of, yeah. Many attempts have been made to stitch Eorzean cultures together with the few things that unite them - faith in Twelve, a shared history, and (after the Autumn War), a currency. But I wouldn't say the city-states are so integrated as to call them a nation (especially in the "it's a a country" sense).
This is actually how the team described it pre-launch.
Eorzea is heavily based on Ancient Greece (from the city-states to the twelve gods). They actually have a long history of infighting, as well, but Eorzea is based more on the time leading up to the Persian invasion (except here it's the Garleans, who are based on Romans); there's an uneasy peace and sense of cooperation, but they don't necessarily get along all that well left to their own devices, even today.
The cities themselves aren't really based on Greece, though. If I remember right, the trio was supposed to be "something like Venice (marine capital; Limsa), something like Palmyra (trade oasis; Ul'dah), something Mayan (forest nation; Gridania)", sans the cultures themselves. Gridania eventually became something more like a Japanese polis.
E-R-P-D! GET ON THE GR-... G-GET OFF THE GR-... GET OUTTA MY HOUSE!
