General Motors said Thursday that CEO Ed Whitacre will step down from his post on September 1st.  The news came on the same day the auto maker reported a 2nd quarter profit of $1.33 billion.  Whitacre will be replaced by GM board member Daniel Akerson, the managing director and head of global buyout at the Carlyle Group.  Whitacre also serves as GM's chairman.  He'll leave that post by the end of the year.