Wierd problem: anonymous user can view event register page, but not event info page (it is completely blank). Have checked permissions, all ok (if I remove View Event Info, there is an access denied message, but if that permission is present, it is the white screen). Have checked custom fields and profiles. No PHP errors logged, nor any errors in Civi's log. Any other ideas what could be the problem?
Thanks @Fabian and @Aidan. I finally discovered my error. I had stupidly given 'Access CiviEvent' permission (Drupal) to anon users. Removing that cleared the problem. https://civicrm.stackexchange.com/a/25326/7190