"There is absolutely nothing in Barack Obama's sanitized background that would indicate he'd naturally pick up the slang affectation "y'all", a mostly southern or rural piece of American slang."

It's pandering, obviously. Very few fall for it. I remember when the other whitest black man in America, the late-O.J. Simpson, used to launch into his faux urban patois during his post-murder acquittal public persona damage control tours in the late-90s. People didn't fall for O.J.'s act, either.