Does the new Messages app just know if a contact is running iOS 5 so it doesn't have to send a costly text message or do I have to somehow trigger it?
From what I have been reading, iM is the first option. Although if it cannot figure out, SMS will be sent the costly route automatically. No option to stop the message from going out... - kartik vaithyanathan
I dare say it would work off the same set of pipes as FaceTime... since iOS5 works for iPods (without SMS feature) - Johnny
Oh I figured it out. When you can send an iMessage the text input field changes from "Text message" to "iMessage". Neat! - Benjamin Golub
Yeah, it goes by AppleID, and I think it matches phone numbers to Apple IDs in its database to faciliate autodetection. - Kevin Fox