Way to go, US Jump Magazine. Here we were, all speculating and stuff, and you have to come along and get pages scanned showing that the Bahumut countdown that had us all abuzz yesterday was for Blood of Bahamut for the Nintendo DS. It's an action RPG featuring 2D sprites fighting in a 3D environment, with the giant walking city looking fellow in the background, do whatever it is giant walking city looking things do. According to the folks over at Final Fantasy Union, an additional scan depicts two characters, Ibuki and Yui, but suggests that the title could be a four-player affair.
Best of luck on that big old countdown clock, Square Enix. You really need to learn to time these things to occur before the magazine scans hit.
Blood of Bahamut Confirmed For Nintendo DS [Final Fantasy Union - Thanks CockroachMan!]