iMessage is one of my favorite new features in iOS 5. It's simple, fast, and most importantly integrated right into the Messages app, which makes it even more compelling, assuming you have friends with iDevices as well.
If you have an iPhone your iMessages are sent to your phone number by default, which acts like your iMessage ID, if you will. If you have an iPad, you can only receive iMessages sent to your registered email address. But what happens if you want to receive all your iMessages on all of your iDevices?