I'm building an event website. The event runs in different countries each year eg London 2012, New York 2012, London 2013, New York 2013, London 2014 etc. Each event has speakers, sponsors and exhibitors and some of these will be involved in more than one event. I have created a custom post type for each of these event contributors. I would like these custom post types to share categories so a speaker and an exhibitor, for example can be assigned to the London 2012, Hong Kong 2013, New York 2013 category. I then want to be able to list all Speakers from London 2012 using query_posts. How can I achieve this? Thanks in advance!