Square Enix have confirmed that Final Fantasy Dissidia will be a new game for the PSP, which will be part of the 20th anniversary of the FF series (which already includes PSP remakes of the first two Final Fantasy games)
You can check out for yourself the official site Square Enix have opened for Final Fantasy Dissidia but not much is there at the moment. The logo for the game is there along with a tagline of:
"Fantasy is endless" and "Fantasy has no limited form"
IGN report that Tetsuya Nomura is a character designer for the title and also that it may not even be an RPG, as its genre is listed as "Dramatic Progressive Action."
SE have promised to release more details at their upcoming party.
Official Square Enix Dissidia Site
Source:
IGN