The worlds of FFT, FFTA, and FFXII are all called Ivalice, and there are certainly a lot of things common among them, but there are plenty of other things that keep them separate.

FFT's Ivalice had humans, monsters, and some convoluted political strife with dramatic conflicts carried out by armies. FFTA's Ivalice was, most importantly, the product of a fictional storybook setting called Ivalice and Mewt Randell's mind; it had humans, monsters, bangaas, vieras, n'mou, môguri, a single undisputed ruling kingdom, and generally lighthearted competition between clans in search of treasure, with battles almost never ending in real deaths thanks to an omnipresent police force.

FFXII's Ivalice certainly won't be too much like either one of the preceding worlds to bear the name, but so far it certainly seems to have more in common with FFTA than FFT--bangaas, vieras, and judges at least--although definitely minus that whole "all in a wussy kid's head" factor.