enabling ordering of back orders even when not in stock
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http://www.magentocommerce.com/boards/viewthread/17353/P15/
I think, Magento still work with backorders bad. If I select “Enable backorders” and my product Is “Out of stock”, i want to still create an Order. 
But if you enable Backorders, it only means: “Don’t switch product availability to Out of Order if Qty is 0”.

Here is the solution:

in app/code/core/mage/catalog/model/product.php 
in function public function isSalable() (cca. line 1204) 
change 
$salable = $this->isAvailable();
to 
$salable = $this->isAvailable();
        
        if ( $salable < 2 ) {
            $backorders = $this->getStockItem()->getBackorders();
            if ( $backorders ) {
              $salable = 2;
            } 
        }
and in app/code/core/mage/cataloginventory/model/stock/item.php

change if in function checkQuoteItemQty (cca. line 519) 
from 
if (!$this->getIsInStock()) {
to 
if (!$this->getIsInStock() && !$this->getBackorders()) {
Now you can add to cart an order product, which is Out of Stok. But only, if backorders are enabled.