UPDATE 7:35 PST: The Final Fantasy XII website has updated in anticipation of the conference, and revealed some new information on the game's protagonists, plus a brief explanation of the story in the "Prologue" section.
FFXII will return to the Ivalice setting that director Matsuno has previously used in Final Fantasy Tactics and FFT Advance. The details of the world seem closer to the more recent Game Boy Advance title, since the Bangaa and Viera races introduced there return; meanwhile, the two main characters are identified as "Humes," borrowing a page from Final Fantasy XI's classification scheme.
Ivalice is split into the Arcadia Empire, which rules over Valentia, and the Rosaria Empire, which governs Ordalia. The warlike Arcadians have captured Dalmasca, a small country that's strategically located between the two larger continents, and replaced its monarchy with their own men. Ashe, the displaced princess of Dalmasca, begins a resistance group against the Arcadians, where she meets Vaan, a young boy.
The character bios describe Ashe as tough and resolute, whose exterior only begins to soften through her friendship with Vaan. Vaan, for his part, is a "bright, optimistic boy" who used to live in downtown Dalmasca, and dreams of escaping the country as an air pirate.
No details on the battle system or other gameplay have emerged just yet, so hopefully more on that will come out of the press conference later on. If you've missed anything previously revealed, the summary of everything known prior to the formal announcement today is below. Keep watching this space for the very latest!