I've been having the same problem....
iMessage was working fine, and still does with everyone, except yesterday morning, it suddenly stopped working between my girlfriend and I (had repeated delivery errors, resent as a text ) Now it's stuck in SMS Text mode with her and I have been unable to revert it. I've tried all of the solutions offered, ie; network reset, turning imessage off and on along with rebooting between, disabled everything, re-enabled everything. I updated to ISO 7.3 last night and it still hasn't corrected the issue. As of today, everyone else is still working as an imessage except for her. I receive iMessages from her, but mine only respond as texts. To top that, I attempted to respond to a group message where both she and the other recepiant are both iphone users to which all of the previous messages were imessages and it only sends text responses to that thread now. I even tried facetime with her to see if the phone would figure it out, but no avail there either. I can, however, send her imessages using her apple id, but that, in turn, created a new thread altogther.
I am completely stumped....