How do I drop an entire stack at once?
As of version 1.4.6 you can now press Ctrl+Q to drop an entire stack at once
You can go to your inventory screen by pressing E (the new default key), click on the stack of items, move your mouse outside the inventory pop-up, and click.
(Mouse is where the floating stack of dirt on the left is.)
The result!
As Amazed stated, you can also simply close the inventory window while holding the stack of items.
Go to your inventory screen, click on it, and click outside the window.