This was a disturbing article I read, where it seems that Bill and Jimmy made things up, and supposedly Bill later said something pretty nasty according to Chris Doohan, James' son, and confirmed by George Takei.

http://trekmovie.com/2010/12/09/george-takei-william-shatner-refused-to-appear-on-stage-with-james-doohan-at-farewell-con/

I find it hard to believe, considering what lengths William Shatner had gone to repair his relationships. I suspect, if anything, the statement is untrue or was misunderstood, and the reasons for not going to that particular convention were more complex than portrayed.