Check out this video of the M-06 guitar "game" for the DS. It may come on a cartridge, but it's not really accurate to call it a game. It's more of a portable rhythm guitar simulator.
You might look at that game and say "Why not just play on a real guitar?" Well, a real guitar takes up space, and makes noise. Both of which are not bus friendly. And I'm always on the lookout for new ways to pass the time during my daily commute.
The DS of course is very bus friendly. But most of my DS hours are wasted away on time devouring RPGs (which only barely qualify as active entertainment). I certainly wouldn't mind having the ability to do something a little more constructive. This little toy looks like the perfect fit for a commuter that wants to compose on the go.