Today Square Enix revealed there would indeed be a Dissidia: Final Fantasy demo for the PlayStation Portable at the Jump Festa 2008 in Japan.
Dissidia was revealed in May this year in the Japanese magazine called "Jump Magazine". The game is described as a Progressive Action game, and will feature close-range combat. The game is exclusive to PSP.
The Jump Festa 2008 will be held in Japan on the 22nd and the 23 of December. Stay tuned.
Source: Forever Fantasy