Jimmy Olsen is Clark Kent's assistant at the Daily Planet, a red-headed skirt-chasing wimp who really only fit in thirty or forty years ago. He'd be right at home in one of the Hardy Boys books or on Leave it to Beaver, is what I'm saying. With all the aesthetic and thematic updates that the Superman universe will undergo in The Man of Steel, it seems they've found a way to try and modernize this character as well.
Jimmy Olsen is now Jenny Olsen, because they've turned him into a girl. It's a little weird that this imformation would come out now, when filming has been finished for nearly a year, but seems that the people at Warner Brothers have been trying to activly hide this. That might seem a little silly to those outside of the comic book zeitgeist, but people even take this kind of stuff seriously. Remember there was a remarkably big hubub when it was anounced that the Thor character Heimdall would be played by a black actor. Of course, the race change for Perry White in this film has got nothing but a warm reception, so I think we can have faith in the fans of this particular franchise to deal with it in a mature fashion.