The lastest issue of Famitsu (either Weekly Famitsu or Famitsu PSP+PS3 according to our beloved Shuyu) has revealed that Final Fantasy XIII's lead role Lightning will be appearing in Dissidia: Final Fantasy. Small photographs of the magazine have appeared on the internet which look legit, but we'll update when/if there is some proper confirmation. It's also unclear how this will be added. Presumably it will be DLC or be a bonus in the "newer" version being released in japan but without being able to read tiny blurry writing or even Japanese to begin with it's hard to tell. We'll keep you posted.

Famitsu - Dissidia Photo 1
Famitsu - Dissidia Photo 2
Famitsu - Dissidia Photo 3
Famitsu - Dissidia Photo 4

Source: Neveryuna24 (Thanks)!

Update: Turns out it is fake. Someone over at Dissidia-France did it as a "Halloween Trick". I thought this was more of an April Fools sort of thing but props for the effort nonetheless.