iMessage inquiry to ease my mind
If the sending phone has no 3G or wifi coverage it will send as an SMS message. Otherwise it gets sent as an SMS. If the receiving phone doesn't acknowledge the message after 30 seconds (because it doesn't have 3G or wifi coverage, or it's turned off) the sending phone will give up on iMessage and send it as an SMS.

If the sending phone has turned off the setting to "Send as SMS" then it won't send as SMS unless the user manually does it.

EDIT: If the receiving user has Messages set up on their Mac running Mountain Lion or Mavericks, and the phone is out of data coverage, the message could be received by the Mac and not the phone.