The analogy is not exact. Huey Long was by many accounts a brilliant thinker and politician. Former President and U.S. Supreme Court Chief Justice William Howard Taft proclaimed him to be one of the finest legal minds he had ever encountered. Sarah Palin has yet to exude intellectual brilliance.
I still maintain that Palin shares far more similarities to a different Louisiana populist: Mayor C. Ray Nagin of New Orleans. (Chocolate Moose Party! 2012!)
Update 4:30pm: I forgot to add a link to Christopher Hitchens' spot on Palin's Dangerous Populism.