As the unstoppable cyborg in "The Terminator," Arnold Schwarzenegger made "I'll be back" part of the national lexicon. Nearly two years after leaving California's highest elected office, he's making good on the promise in a big way. The former Republican governor, who just turned 65, told The Chronicle that he will head up the Schwarzenegger Institute for Public Policy at University of Southern California, a think tank designed to explore "post-partisan" politics and global challenges. Among the luminaries working alongside him are former Mexican President Vicente Fox, Austrian Chancellor Werner Faymann, former U.S. Housing and Urban Development Secretary Henry Cisneros...