Roberto Orci, writer for The Transformers movie, has posted a message on Don Murphy’s boards, stating that Arcee is not in the Movie and the reasons why
I hope that all those who hate Arcee will be gracious to those who love her. And to all those who wanted to see her, you should know the decision was not based on a mere dislike of the character, nor an attempt to defeminize the franchise. The decision is, at this point, story driven.