From rpgamer:

The Kingdom Hearts 3D: Dream Drop Distance demo at Tokyo Game Show this year had a surprise for attendees. This comes in the form of a character that is not from a Final Fantasy game, a Disney title, or an original conception, but instead is the lead in another game conceptualized by Tetsuya Nomura: Neku Sakuraba from DS game The World Ends With You. The TGS demo has two parts and during Sora's episode, he calls out for Riku. Instead of a reply, he finds Neku hanging from a roof. The boy jumps down and tells Sora that he needs a partner to play the Reaper's Game, showing him the timer on his hand. Whether there will be more The World Ends With You characters or references in the game remains to be seen.

A trailer shown at TGS also gave a more specific release date. It will be out in Japan in the spring of 2012. Release dates for the rest of the world are not yet known. Kingdom Hearts 3D: Dream Drop Distance will be the first entry in the series for the Nintendo 3DS. It will reportedly provide a glimpse of Kingdom Hearts III, which has yet to begin development.