Does iMessage fallback to SMS on a per message basis? As we know, the messages app in the iPhone will detect if the party you are texting to is iMessage compatible or not, and will then send an iMessage instead of an SMS. This is great, and works as advertised with other iPhone users that have data plans.

However, what happens with an iPhone with NO data plan? (bought it unlocked, for example). The ideal situation for me, would be that the message is first tried to be delivered as an iMessage, and after some time it would time out and be delivered as an SMS instead, but I don't think that's the case.

Does anyone has experience with this? Will iMessage fall back to SMS on a per message basis?
Yes, it will.

I've experienced it a few times, if the message stays undelivered for something between one to five minutes (I haven't checked exactly), it will fallback to SMS.

This behavior can be changed in Settings > Messages > Send As SMS, checking it off will not send the message as SMS if iMessage is not available. Rechecking it will send the message not delivered (even if they were sent before).