It is not confirmed nor is it even being planned at the moment. But a sequel to Kingdom Hearts 2 is very likely do to the sales of Kingdom Hearts 2 right now. From the information that is floating around, I really think that Kingdom Hearts 2 is the hottest game around right now.
Nomura also stated that he would get the team back together and they would make a new Kingdom Hearts's game determining the sales of Kingdom Hearts 2 in its first year, since Kingdom Hearts 2, pretty much sold the most amount of copies in the first quarter of 2006 in America.
It also sold well in Japan. Nomura has some thoughts about the next Kingdom Hearts's game, he said that it would not be a direct sequel to Kingdom Hearts 2, meaning that it would not follow a direct story like the others, this does not mean that it would not be a sequel though, It just might not be called Kingdom Hearts 3, it would still be called Kingdom Hearts, just with some thing else attached to its title.
If they where to make a next Kingdom Hearts , I belive that Square Enix would begin to work on the the next game sometime in December of this year. But things are not as they appear so we can't expect to much intill more information arrives.