Nintendo just released the first proper details of The Legend of Zelda: Spirit Tracks, following this morning's showing of the DSi game in GDC's Iwata keynote.
Via press release:
The new The Legend of Zelda™: Spirit Tracks game that Iwata announced will utilise the intuitive touch control of Phantom Hourglass in an all new adventure that will please longtime Zelda™ fans while keeping the game accessible to all DS owners. The concept behind this Zelda game includes utilising a steam locomotive to travel around the world, switching Phantom and Link to explore the dungeons, and solving puzzles using new items.
It's out this year. There's pics on Joystiq, if you're interested.