Configurable products in Magento are difficult to handle, as they consist of multiple “Simple” products that have their own stock levels, prices etc.
These notes talk through how to add “Out of Stock” options to each of these on the Magento product page, and some extensions to provide Out of Stock Notifications when they are:
Programming: Show out of stock configurable options
http://www.mindgeek.net/magento/magento-show-out-of-stock-configurable-options/
Programming: How to show out of stock configurable options with Magento
http://www.ayasoftware.com/content/how-show-out-stock-configurable-options-magento
Extension: Aheadworks – Product Updates Notifications ($89)
http://ecommerce.aheadworks.com/magento-extensions/product-updates-notifications.html
Extension: Amasty – Out of Stock Notifications ($79)
http://amasty.com/out-of-stock-notification.html
Extension: Apptha – Out of Stock Notification ($79)
http://www.apptha.com/category/extension/Magento/out-of-stock-notification