iMessage automatically uses SMS for all devices not registered in iCloud. What it will do is eliminate all messaging charges between newer iPhones. This is a significant percentage. Competition will take care of the rest.
I believe Windows Phone's latest update (Mango) will also use something like this but it will automatically jump between Windows Live Messenge, SMS, and Facebook Chat.
WebOS was the first to have just a "Messages" app (I believe it worked with SMS, AIM, ICQ and GChat/GTalk) that allowed you to have a conversation between a person over different protocols/services, but it was not automatic. Not sure if this has been changed for WebOS 3.0