I really think that they pretty much are boyfriend and girlfriend at this point.
"Kairi! Remember what you said to me? I'm always with you too. I'll come back to you. I promise!"
That makes me think they are already together. They held each others' hand while the world was splitting apart in the end cinema. I can totally see one of them saying "I love you" or something similar to that. I don't think a kiss would happen, unless it's on the cheek, but I think at the end of KH2, they will finally be reunited, and be a big romantic scene where they go hug, and hold each other for a long time, and say "I missed you so much" or "I knew you'd come back to me Sora."