Newsweek. Versions are also on their way to PC and Mac, where the test prep software is already fairly common.
The project is early in development and does not yet have a name. Newsweek reports that it was Aspyr who pitched the idea to Kaplan. If this is a success, can we get a DS game to help us understand Special Relativity? Perhaps we can get Professor Layton or perhaps even Wario as our virtual teacher.