If you try to print an actual packing slip form it will not contain back-ordered quantities. To work around this you can customize an invoice form to replace the packing slip.

1. Customize an invoice (Setup > Customize Forms > Invoices). The sales layout should be "Item."
2. Delete the 'Sale Status' field from the form
3. Add a text field with the words 'packing slip'
4. Save this as 'packing slip'.

When you're ready to print a packing slip, go to Print Invoices instead, and print your 'packing slip' invoice. It will contain the backordered quantities.