How to enable Back Orders in Magento:
To enable back orders,
go to:Admin Panel -> System -> Configuration -> Catalog Tab -> Inventory -> Product Stock Options -> Backorders = Allow Qty Below 0 and Notify CustomerEdit Product to Qty = 0 and Stock Availability = In Stock
When you add the particular product to cart, the product is added to cart and you will see the following message:
"This product is not available in the requested quantity".
This code is very useful for me during my development of Magento out of stock Message.