Small zombie attacks have occured in the past, although no full-scale apocalypse has ever happened. it is likely, however, that a zombie apocalypse will occur within the next 2-10 years, so be prepared! More info can be found aroud the internet, and in Max Brook's book, The Zombie Survival Guide.
Well, if it really worries you, drive to a grocery store and board yourself up. That means you will be unseen and will have a large food supply. A zombie apocalypse has never happened in the history of life, so it must be long overdue. Get ready!