Longtime series artist Yoshitaka Amano recently stated in an interview with a French magazine that among his current projects is Final Fantasy XIII. It is unknown exactly what role he is taking in the development of the next game. Up until FFVII, he'd served as a character designer for Square, but since then has primarily done logos and supplemental artwork based on other designers' work (such as Tetsuya Nomura). His other projects include an animated film, The Planet of the Winds, and other games yet unannounced.
Either way, this seems to confirm the existence of the thirteenth in the long running series.