Two excellent resources for ramping up on SharePoint 2007 event receivers.
http://blogs.msdn.com/brianwilson/archive/2007/03/05/part-1-event-handlers-everything-you-need-to-know-about-microsoft-office-sharepoint-portal-server-moss-event-handlers.aspx
http://www.davehunter.co.uk/Blog/Lists/Posts/Post.aspx?ID=69