I just updated both of my iPhone 4S (Sprint) to iOS 5.1.1 and both of them started doing this very peculiar thing with iMessage
Sometimes iMessages just fail to send, its life, I understand that. However, ever since this last update iMessages have re-sent on their own as SMS. No action from me, I don't mind if it's here in the USA, but when it switches to SMS on my Australian friends, that's kinda annoying!
Send as SMS is turned off on both (never turned it on) they're brand new, and I have set them both up as new phones in iTunes and only loaded apps and my iCloud contacts/notes/calendar/mail.