It's not a game, precisely, though you do play with it (or on it, or through it...). Essentially, it turns your DS into a music creation tool -- but that's not quite right either, since you create rich and colorful visualizations as you play, and that's a huge part of the experience.
A fellow CMS student (Tracy Daniels) showed our class a video of an MIT Media Lab demo that sounds kind of similar to this, in which users could create beats and music by moving discs around a projector-illuminated table. Still, the idea of being able to do something like that on a $130 handheld with a stylus sounds pretty darn cool, even if the feature set will inevitably be somewhat constrained.
Update: Jeremy Parish of 1up.com argues (quite convincingly) that Electroplankton should put the inane debate over whether games are art to bed once and for all.