iMessage is a built-in capability within iOS5 which allows users to exchange text messages with other iMessage users on their iPhone), iPad and iPod touch devices. You can start a conversation on device and pick it up on another. The handy part about this nifty little feature is that if you're out and about texting on your iPhone with another iMessage user, when you get home you can pick up the conversation on your iPad and take advantage of the larger screen (and keyboard!).
There seems to be one glaring omission in the multi-device love that iMessage is spreading around: Apple's line of desktops and laptops. It would be amazing if Apple allowed users to continue a conversation already in progress on a computer, rather than just limit them to a mobile device like an iPad or iPhone. What's more, imagine having the ability to start an iChat conversation with a friend and be able to continue that conversation on your iPhone? All Apple would have to do is to add a service to iChat that would allow iMessages to be sent to a desktop or laptop in addition to your favorite mobile device.
When FaceTime first launched, within a short timespan, Apple quickly realized that it needed to bring that capability to the desktop and laptop arenas. Doing so gave the same experience unilaterally across its line of devices. If Apple were to do the same with iMessage, it could breathe new life into a stale instant messaging world.
As a side note, since most of my friends have iPhones are using iMessage, I'm considering changing my the texting plan on my AT&T plan. No need in paying for more than I should be.