One of Nintendo's several surprises at this year's E3 Media Briefing was the announcement of Golden Sun DS. There wasn't any real information given during the briefing, but the game was technically "playable" on the show floor. The playable portion only lasted about half a second. You start off in a canyon-like environment, and as soon as you move to the right, the screen starts showing you the E3 trailer.
The art style and graphics looked great, with a black-outlined/cell-shaded look on the characters and environments. The graphics move into a high-quality, traditional full-3D when in battle. The interesting part was that the Nintendo rep wanted me to answer a few questions in a mini-survey style, since I hadn't played the first two games. It seems they're soliciting opinions on the art style for the game, indicating perhaps that they're not too sure of the direction themselves.
There was no narrative information given, and no release date mentioned, though the game didn't look incredibly far along. In the meantime, check out all the screenshots that Nintendo did release, as well as the trailer, and see what information you can glean from that.