^ Yeah, it's in her autobiography. She claims that she was the only other cast member to receive as much fan mail as Shatner and Nimoy, but that because of studio racism, the people in the mail room were ordered not to deliver it to her. It was supposedly only because of a courageous young man in the mail room who informed her and invited her down to receive her mail that she ever found out.
Regarding the MLK story, I wouldn't say that it couldn't have happened. Whether or not the character of Uhura was as historically significant as Nichols likes to say, it's still true that she was an black actress playing a significant, non-stereotypical role as an officer and a member of the command crew. While there are other examples from the time period that can be cited of black actors, as well as women, in non-stereotypical roles, they were certainly not the norm. I could understand if MLK had encouraged her to continue.
None-the-less, her story has changed several times over the years. If I am not mistaken, it began as her simply being inspired by MLK that provoked her to continue. Then it was a phone call. Then it was an in-person meeting where MLK requested to meet her. The fact that it has changed so many times over the years does stretch her credibility on the issue, but I suppose we'll never know for sure.
(BTW, the whole thing about Star Trek having the first interracial kiss was bunk too. Lucille Ball and Desi Arnaz anyone? Yeah, yeah, I know they mean specifically a kiss between a white person and a black person, but that's not the only thing "interracial" means.)