Modify Alert Notifications using AlertTemplates.xml in SharePoint 2007
http://www.combined-knowledge.com/Downloads/Modify_Alert_Notifications.pdf

How to change the Alert Email notification message for Announcements
http://dirkvandenberghe.com/archive/2008/01/08/how-to-change-the-alert-email-notification-message-for-announcements.aspx

Customizing Alert Notifications and Alert Templates in Windows SharePoint Services 3.0
http://blogs.msdn.com/sharepointdeveloperdocs/archive/2007/12/07/customizing-alert-notifications-and-alert-templates-in-windows-sharepoint-services-3-0.aspx

SharePoint Alert customization
http://sridharu.blogspot.com/2008/12/sharepoint-alert-customization.html