[shirkey]

> Message can intelligently switch between Apple's own backend and SMS depending on data connection and presence information

I'm not familiar with the Apple implementation, but something similar was implemented by the Yahoo Messenger client app running on my Windows Mobile several years back, seamlessly switching from Internet data connection to SMS -- resulting in an SMS for every single outbound message and a bill of around $300 from AT&T for all those "OK" and "Sure" messages.

[mikeash]

Fortunately for those of us without SMS plans, iMessage lets you turn off the SMS fallback, and displays a clear visual distinction when it gets used.