Square Enix has registered "Dissidia" in the United States Patent and Trademark Office on April 6. The usual rumors are not late to come: French website Final Fantasy World claims "Dissidia" might be the 5th installment in the Fabula Nova Crystallis franchise because it is a Latin word. The Fabula Nova Crystallis franchise already includes Final Fantasy XIII, Versus XIII, Agito XIII and Haeresis XIII.
Forever Fantasy did their own researched to validate the registration of "Dissidia" in the patent office by Square Enix and found out it's true (You can view the screenshots using 'Read more').
Perhaps the "New Title Announcement" in the upcoming event, Square Enix Party 2007, will clear things up. Only time will tell.