One of the cool things that iOS 5 will offer when Apple releases the new mobile operating system is the capability to create custom shortcuts that can be used when entering text. Of course you can have this feature right now if you jailbreak your iPhone and install the utility called Xpandr using the Cydia app store.
Xpandr costs $1.99 and can be found in Cydia under Sections -> Utilities or by using the Search function. Payment is simple and uses either PayPal or Amazon Payments directly in the Cydia app store. Once installed, Xpandr can be configured by tapping its icon on the home screen.