The Siliconera writer explains that during Sora's half of the demo in Traverse Town, Sora stumbles upon Neku while searching for Riku, who, and I quote, "..jumps down and shows Sora the number on his hand." The site then mentions that Neku explains that he is in need of a partner to play the Reapers game. Which I'm sure fans of the series will understand the meaning of.
It should be noted that this is the very first time that a cameo has been made in a Kingdom Hearts game by a character not of the Final Fantasy family. It should be quite interesting to say the least.
