Perhaps Square Enix are hoping that fans see the vast improvement between Final Fantasy XIII and Final Fantasy XIII-2, the same way they saw the difference between Assassin's Creed 1 and 2, perhaps they're just fans of the series; however it came about, there's going to be a little bit of Assassin's Creed in Final Fantasy XIII-2 when it comes out early next year.
All we really know about this is that it exists – it has been officially announced by Ubisoft and Square – and that it'll be available as DLC as a costume for new character Noel. No word on price, how long it'll be before it's available after release or if we can expect any other little cameos, but for now it's a cool little reference that they'll probably charge far too much for.
Ezio, the hero of Assassin's Creed 2 through Revelations, was recently revealed to be a fighter in the next Soul Calibur game.