I could be misremembering, but I do recall Abrams having said that they couldn't find a way of putting Shatner's Kirk into the story which fit to anyone's satisfaction. If there was a actual part written, it would be interesting to find out what they tried.Some time back, I read on trekmovie.com that a part in the story had been written for Shatner's Kirk, but it didn't fit in too well and made the story too cumbersome. If I remember correctly, Abrams said that this part would be revealed after the movie was released.
Actually, a very quick and search of trekmovie.com, and I found the story. It was October 14, 2008. Here's the pertanent bit from the story as it was reported on the TrekMovie site:
Last month JJ Abrams told AMC that there was a scene written for Shatner and then a few weeks later William Shatner did a videoblog about how no one ever offered him a role. Although this appeared to be a contradiction, TrekMovie noted at the time that they could both be telling the truth. Today in a new interview with MTV Abrams clarifies that he felt the scene written by Roberto Orci and Alex Kurtzman was 'good' but was also a 'contrivance' to get Shatner into the movie and that it didn't work, and so they never offered it to Shatner directly.
by the way, writer Roberto Orci has promised to share the Shatner scene with TrekMovie readers, but after the film opens.