This could be done using http://drupal.org/project/workflow
So, a user fills out the event details form. When they hit submit, it goes into a event submission queue and the event co-ordinator automatically gets sent an e-mail. They then call the person who submitted said event (we ask for their contact details, event flyer/poster image, et al on the form). If they confirm that the date/time requested is available (or can arrange a different time/day), that the event is workable and whatever else is required to be checked, then they go to the event queue and change the event from a draft into a confirmed/published item and it goes live on the site.