The word comes from Film Chronicles, who don't state their sources but do throw out the suggestion that this is the reason Cumberbatch exited Guillermo Del Toro's horror film, Crimson Peak. At the time we were told specifically that his departure had nothing to do with another project, but that could have been a lie designed to lead Internet sleuths in another direction. Both films are scheduled to film at pretty much the same time next year, so there's no way Cumberbatch could have done both.
So this leads to the question of who Cumberbatch could play. Popular opinion is that the film will take place roughly thirty ears after Return of the Jedi and follow the Solo and Skywalker children. It's possible Cumberbatch could be one of them, but there are a couple of older characters that would make just as much sense, including one who was described as a "thirtysomething male, intellectual" in the released casting sheet. If that doesn't fit Cumberbatch to a tee, what would?
Last week it was Alex Pettyfer and Rachel Hurd-Wood who became the latest names mentioned as possibilities, but other rumors have associated Dane DeHaan, Leonardo DiCaprio, Ryan Gosling, and others. Until an official announcement comes from Lucasfilm, take it all with a grain of salt. But right now, it's looking pretty good that Cumberbatch will be on some side of the Force when Star Wars: Episode VII arrives in 2015.