We already know that Final Fantasy XIII, Final Fantasy Versus XIII for PS3 and Final Fantasy Agito XIII for the PSP are all part of the Fabula Nova Crystallis universe. Square have previously mentioned that Versus and Agito are not spinoffs, but rather a different story in the same universe.
However, it seems that the Versus XIII and Final Fantasy XIII stories may overlap and be closer than we first thought as IMDB's listing for <a href=http://www.imdb.com/title/tt0960140/>Final Fantasy Versus XIII</a> suggests that at least two of the Englsh voice actors from Final Fantasy XIII will also be recording voices for Versus XIII.
The page suggests that Daniel Samonas will be recording a voice for Maqui (pictured below), who is part of Snow's gang in Final Fantasy XIII.
It also suggests that Erik Davies will be recording a voice-over for Cid Raines (pictured below), just as he did in Final Fantasy XIII.
Of course this is still classed as a rumor at the moment, but the page was updated fairly recently (April 26th 2010), so could be accurate. If it is true however, we may even be seeing some of the more major characters such as Lightning in the game, great news for Final Fantasy fans.