Another suggestion has been added to the Quantity Discounts FAQ:

http://www.thatsoftwareguy.com/zenca...discounts.html

If you are concerned that people don't know they might have a discount coming
when they look at their cart, try changing SUB_TITLE_SUB_TOTAL in includes/languages/english/shopping_cart.php to something like 'Sub-Total BEFORE Discount'
This will emphasize the fact that a discount will be added at checkout time.

Good luck,
Scott