There seem to be people who think that the last episode of ENT implies that the entire series was a holodeck fiction in the Star Trek continuity. Nope, Riker was just watching a holodeck representation of FACTUAL history, and at a certain point he switched to interactive adding only fictional interaction, much in the way we read a factual history book (non-fiction), in which we read added footnotes with the vision of the writers.
Pamela: Oh Bobby, in my dream you have been dead for over a year now.
Bobby: You should stay off the sleeping pills.
Pamela: Really Bobby, in my dream you drowned in the swimming pool.
Bobby: You should have known better Pam, or have you forgotten I can't drown. I'm the man from Atlantis, for crying out loud!
I suspect that if Shatner and Nimoy do appear in the movie, they will NOT play Kirk and Spock! They will have some sort of cameo like Charlton Heston had in the reboot movie of Planet of the Apes. Like Roger Moore had in reboot movie of The Saint, like Patrick McNee had in the reboot movie of The Avengers. Like Lou Ferrigno in the reboot movie of The Hulk. Like Richard Hatch had in the new Galactica series, ...
Every reboot always wants to make an homage to the original they were based upon by having some of the original actors do a cameo. Exept when you have an egomaniac (hope he won't sue me) like Tom Cruise producing the movies in which he plays. He must have thought a cameo of one of the original MI actors would have taken him out of the spotlight.
Ironically of all the examples I mentioned above, only MI seems to have been succesful. Yet I still believe rebooting the franchise is the only way to go when your franchise was killed by too much continuity.