Henry Brown

Bruce Schneier would be my FIRST choice
Not sure of the current status of Richard Clarke, whether inside or outside of government, but IMO he would be close to the top of any list of keynoters
Another excellent speaker, like Mr. Clarke not sure of his current status, would be Howard Schmidt
Others could include:
Alan Paller of the SANS institute
Robert Dix of Juniper networks
Any of RSA management/speakers