According to http://www.scribd.com/doc/123929134/Justice-Department-Memo-on-Legal-Cas..., the Department of Justice has written a white paper affirming some circumstances under which it would be acceptable to assassinate an American citizen.
We call upon the President and his administration to reject this white paper and publicly affirm that any assassination of any American by the U.S. government is unthinkable and unacceptable.