Glad you linked that, Thalzon. As soon as I saw the title of this thread I thought it was misleading.
Square Enix's new trend is that Final Fantasy games come in series.
Right now, with Final Fantasy XII, we're seeing Ivalice Alliance, which is basically the main FFXII game(for PS2), FFT Lion's War (for PSP), FFT A2 (for DS), and FFXII Revenant Wings (for DS). They all take place in the same world and are linked together. (Technically FFXII for PS2 isn't part of IA, but it's still an Ivalice game with links to the other games).
The main Final Fantasy XIII game is going to be on PS3. FFXIII Versus is also on PS3. There are still other games that are going to be on other systems that we don't know. Probably one or two of them will be on a portable system like DS or PSP, and there's probably a cell phone game in there because they like those over in Japan.
So yes, the Final Fantasy XIII series is going to be multiplatform, but the game Final Fantasy XIII is not.