Here is the story: "Sid Sheinberg will deny this story because he doesn't remember it, but it's a true story," Gale recounted. "Sid Sheinberg didn't like the title Back to the Future. Every other executive at Universal thought it was a great title, as did Steven, as did we. And Sheinberg would not get off [the idea] that Back to the Future was a bad title. He said, 'It's not hip, like Ghostbusters was hip.' So he sent us a memo, and he said, 'My suggestion for the new title of this movie is Spaceman From Pluto. Here are some notes I have regarding the script and how we can make reference to this in the movie.'
"There's a scene in the movie when the DeLorean is in the barn and the kid has a comic book, and it's called Space Zombies From Pluto. Sheinberg said, 'Change that to Spaceman From Pluto and have the kid say, "Look, it's a spaceman from Pluto!" And in the scene when Marty intimidates George McFly by saying, "My name is Darth Vader--I'm an extraterrestrial from the Planet Vulcan," have him say that he's a spaceman from Pluto.' We were saying, 'He's serious about this, Steven. What do we do? We don't want to change the title of this movie.' Steven said, 'Okay, I know what to do.' So Steven dictated a memo back to Sid, and the memo said something like this: 'Dear Sid, Thank you for your most humorous memo of such-and-such a date. We all got a big laugh out of it. Keep 'em coming.' Steven said, 'Sheinberg will be so embarrassed to tell us that he was serious about this that we'll never hear from him about it again.' And he was right."
It's on the DVD commentaries (thats where I first heard it) but I got that information from this website because I didn't feel like typing it out :D:
http://www.wga.org/WrittenBy/1200/secret.html