The description of the setting Configuration->My Store->Display Prices with Tax in Admin (DISPLAY_PRICE_WITH_TAX_ADMIN) reads:
Code:
Display prices with tax included (true) or add the tax at the end (false) in Admin(Invoices)
but it's far more reaching than just the invoices. The function zen_add_tax (present in general.php) determines whether or not to add the tax based on this setting and that function is used in the admin-level base order processing (details, invoices and packingslips) as well as some plugins that have followed the lead of the /admin/orders.php.

Anyway, with that configuration value set to false, one result is that when the order's details display of the purchased products, the value for a product's price (inc) and (ex) tax are equal because the tax is being added ... nowhere (well, other than the order-total itself).

Was this side-effect intentional?