As you can see by my responses [below], everything you’ve read at this point has been rumor and misinformation. As of this moment, to my knowledge, they have not cast Megatron and I am still very much in the running.
Frank Welker talks about the role of Megatron in the upcoming Transformers movie at http://mkbmemorial.com/FWHp/fwhp_Q-and-A.html