TV Guide has been updating all day from the yearly Television Critics Association panel. Today it has been officially announced by DC Comics CCO Geoff Johns that the Arrow/Flash universe on The CW will indeed stays separate from the Zack Snyder Batman v Superman universe in the movies. The reason being pretty obvious; that they don't want the TV writers to be a slave to the movie universe:
"You'll see a lot of DC universe characters [on The Flash and Arrow]. You won't see Batman or Superman. We're on production on Batman V Superman now. So you'll see characters like The Atom or Firestorm, but no not Batman or Superman right now [on TV]. It's a separate universe than film so that the filmmakers can tell the story that's best for film. While we explore something different in a different corner of the DC universe. We will not be integrating the film and television universes."
Now I will point out just because Arrow won't be connected to Batman v Superman does not mean Stephen Amell and by extension Grant Gustin (The Flash) can't be in the Justice League. Now hear me out.....What is one of the most famous comic events DC has put out? Crisis on Infinite Earths. Basically their is multiple earths, with multiple characters just on different paths in the comic worlds. So hypothetically speaking if Zack Snyder wanted Amell and Gustin to play Arrow and The Flash in Justice League he could. They would simply be playing another version of Arrow and Flash in a separate universe! While still playing another version on TV. But that's just a fans dream on something will will probably never happen...
Batman v Superman hits May 2016
DC's Geoff Johns sayst DC wants to keep its film and TV universes separate. Doesn't want TV writers to try to fit shows into the movies
— TV Guide (@TVGuide) July 18, 2014