They never came out and 発言しました they were together. The レポート where Rob confirmed them to be a couple was false. A fabricated interview.
So, the answer is no. They are not together... from what they have said, they have never 発言しました yes.
However, the many, many attempts to evade the 質問 によって both Rob and Kristen hint of their relationship. I'm convinced that they are, but I don't want to spread rumors around によって saying they are when they themselves haven't confirmed.
I don't think THIS in particular has to do with them being together. It's just that they've sort of made it a I guess あなた could say tradition for the winners of Best キッス to actually キッス when they do their speech. For Example, my most memorable one was Ryan ゴスリング and Rachel McAdam's kiss, they did it exactly like they did it in The Notebook. But I can see how あなた would think that because of the rumors.
Not sure if they are a couple, and frankly i don't really care xD. I guess they kissed out of respect maybe for the awards. Would be pretty stupid if they didn't キッス after winning the best kiss-award two times in a row xD