Microsoft have started the Microsoft Dynamics GP 2013 Feature of the Day series on the Inside Microsoft Dynamics GP Blog.
The thirty ninth feature they've announced is Update Back Order Item Qty In Field Service.
Microsoft Dynamics GP Service Call Management and Depot Management will now update the Item Quantity Master when they have a back ordered item. This will be visible in the Item Inquiry and Item Quantities Maintenance windows and give you a truer representation of your inventory quantities.
The example screenshots show Item 3-D2657A (DB 15 Male Adapter) with no items on back order in Item Inquiry;