As you will almost certainly know, Final Fantasy VII: Dirge of Cerberus is part of the FFVII compilation that Square-Enix are releasing.
Dirge of Cerberus is thought to take place a year after the movie Final Fantasy VII: Advent Children ends, and will star Vincent Valentine who was merely an optional (albeit highly regarded) character in the original game.
The game also seems likely to feature Cait Sith from FFVII. Cait Sith seems very likely to appear in Advent Children and therefore his role in both game and movie are eagerly anticipated and confusing. It is unknown whether or not others from FFVII will play a role; it is widely rumoured however that Reeve will return, as would make sense given his involvement with the aforementioned Cait Sith.
There is very little other than speculation circulating with regards to the storyline of Dirge of Cerberus; it is likely we’ll learn a lot more about Vincent and his background, which was partly enigmatic in the previous game.
We will obviously upload more information on Final Fantasy VII: Dirge of Cerberus as soon as we get it, though clearly at this stage there is very little confirmed, even on the Square-Enix Website.