I have worked with Commerce Server 2007 since back when it was called Commerce Server 2006, but with enough projects between then and now, including a swanky WPF application, to leave me pondering over problems I've faced before. So I decided that what I need is some sort of web based log of past technical experiences.
Today's deja vu moment was while trying to set up an inventory catalog to allow backorderable products. I knew it was just a matter of configuration, but it still took far to long to remember just what needed to be configured, and to what. So, if in a few months time I have the same problem, this is how to do it...
Backorderable - "A SKU that can be back-ordered; that is an order can be placed even if the OnHand Quantity is less than the Effective Stockout Threshold. Back-ordering occurs for items that are already selling and have gone out of stock."
Needless to say you shouldn't be doing any of this directly on a production environment.
Step 1
Open Commerce Server Manager and navigate down to the inventory resources for your site. Right click on Inventory, select properties, and change the 'Stock Handling' property to 1. This will change stock handling to be based on stock type, as opposed to the default stock handling, and will prevent backorderable products from being reported as out of stock. While you're here make sure that inventory is configured to display backorderable items (on by default). Any changes to the properties do not take effect until you 'restart all services that use those values' <--- IISRESET if you're on a development machine.

Step 2
Open Catalog Manager and find a product that needs to be set as backorderable. Edit the product and switch to the inventory properties tab (variant inventory properties if you have variant products). The properties you're interested in on this screen are:
Backorderable – Boolean field that sets the product to be backorderable. This needs to be ticked (obviously)
Status – Controls whether the inventory properties are used or not. Set to Enabled
OnHand Quantity – Current stock level. Set to 1 for testing
Backorder Limit – Must be a positive number defining the maximum backorders allowed
Stockout Threshold - A value that the quantity must be above or equal to for the product to be considered in stock

After saving the changes you should now be able to backorder this product on your site, and the process is the same for preorder products to.
To make sure that everything is working you can use the inventory properties to check that backorders are being processed correctly as shown below. After ordering 5 of this product you can see that the available quantity is now -4, and the backordered quantity is 5:
You can also look at the line item of the resulting order in the Customer and Orders Manager to see the backordered status:

Simple when you can remember what needs to be configured!