Leonard Nimoy has ended up being integral to the Star Trek reboot films, but William Shatner hasn't come near them yet. There was a cameo written into the first film for him, but it was dropped before shooting, and there was no place for him in Star Trek Into Darkness. Shatner himself has been vocal about wanting to appear in one of these movies, and it seems like with professed Trekkie Roberto Orci helming Star Trek 3 he may be getting his chance.
I have learned that the script for Star Trek 3 includes a scene that reteams Shatner and Nimoy onscreen as Kirk and Spock for the first time in canon since 1991's Star Trek VI: The Undiscovered Country. It would be a pretty huge moment for fans of the franchise, and likely the last time we'll ever see Shatner as James T. Kirk in official continuity.
Will it happen? The movie is still a ways out from shooting, but unlike the cameo intended for 2009's Star Trek this appearance is plot-driven and integral enough that it might just survive any future revisions. And if it does, will Shatner - who can be prickly - agree to the appearance?