In an interview with MTV News, Star Trek co-writers Roberto Orci and Alan Kurtzman reveal William Shatner almost got a cameo appearance in the new film, the way his co-star Leonard Nimoy does (warning: plot spoilers ahead).
According to the writers, Shatner was to appear in a hologram that the elder Spock (Nimoy) uses to convince his younger, skeptical self (Zachary Quinto) that he and Kirk really are destined to become the best of friends. Here's Kurtzman describing the hologram:
"It was a 'happy birthday' message [with Kirk saying], 'This is the last time I'm going to be able to wish you happy birthday, so I want to tell you how much you've meant to me and how amazing it was that we had all these adventures together... That narrative, that voice-over, became a link [to be heard] over [scenes of] this new crew coming in ... a young Kirk accepting the medal and becoming captain of the Enterprise."
Sounds cool, no? Unfortunately, other people in the filmmaking team (presumably director J.J. Abrams) nixed the idea, claiming it felt too contrived and that the scene wasn't meaty enough to "waste" the actor's time. Maybe so. But it still would have been awesome to see Shatner's mug in the film, if only fleetingly.