iMessage inquiry to ease my mind I have discovered that at certain times, my iMessages are delivered as SMS instead; which is perfectly ok with me, considering I have it set up to do so.  My question is, under what conditions are the messages sent as SMS with both myself and the recipient does not necessary been in a wifi area to receive or send iMessages...is it that when the recipient has their phone powered down that it is sent as SMS, and if that is the case...why is it that I receive messages sent as SMS instead of iMessages from the same recipient...what exactly does "...when iMessage is unavailable" mean??? Thanks in advance for any/all response(s)...
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.