Just type in the word or phrase that deserves a shortcut, and then plug in the shortcut itself. Now, for example, when you type in “fwiw,” iOS will provide an auto-correct prompt for “for what it’s worth.”
…And the fun doesn’t end there. I see great potential for mischief too: Grab your pal’s iOS 5 device, and begin entering shortcuts for common words like “hi” or “siri,” as in the screenshot above. Hilarity abounds as your friend suspects his or her iDevice is suffering demonic possession.