I'm in the exact same situation. Messages to my wife (att iphone) all fail unless I turn OFF iMessage. Texts to everyone else seem to work fine. We did not change any settings prior to this issue. I've done all the upgrade, sync, reboot stuff,...no luck.
I am having similar problem , texts arriving late to my bf,however he does have imessaging on.
I did the updates and he did not I wonder if that matters.
The texts that dont go as imessage, resend themselves as text messages.
not sure how to fix at this point.