You're right. They have never kissed. I'm a Kataang fan, but I have to alisema that they were so close on The Crossroads of Destiny, the were so close that they look just like a couple :S... I hate that. ¬_¬
No. They never kissed, but Zuko was meant to be a upendo interest for Katara along with Jet, Haru, and Aang. The Zuko idea fell through in the end, they have their moments of friendship, but nothing more.