This week's Shounen Jump magazine reveals that Square Enix will be including in each copy of Final Fantasy XIII a campaign code for next year's Final Fantasy XIV. The code will ship with the first print run of FFXIII (which will presumably be in the millions of units).
Just what the code does is a mystery at this point, but it is certainly surprising to see Square Enix pair two different numbered FF games like this.
Jump also offers a short glimpse at an expected feature for Final Fantasy XIII: Chocobos! You'll be able to ride the FF series mascot characters as you explore the vast fields of the Pulse lower world. Chocobos have a life meter which depletes when you run into an enemy. When this reaches zero, the Chocobo runs off, and you're left on foot.
We should be getting a closer look at FFXIII's Chocobo system next week, so be sure and check back.