As the name suggests, the new game is a "concept remake" of Castlevania: The Adventure for Game Boy; a game that desperately needed to be remade. ReBirth will have new enemies and altered level designs -- most notably, stairs have been implemented to replace the ropes from the Game Boy game. It'll also have new arrangements of classic Castlevania music. Like the other ReBirth games, this one is going for a faithful 16-bit look.
ReBirth will be out in Japan on October 27 for 1,000 Wii Points. Nintendo's calendar gave it a "Q3 2009" date for North America, so it could be out here by Halloween as well!