Western developers like the DS

[A Black Falcon]

The GBA Boktai is the game with the light sensor, remember? The cart has a light sensor that senses sunlight (not lamp light, sunlight only!)... your gun charges when in sunlight, when in the light some areas are brighter, etc... it was an experiment with a unique gameplay device (though the gameplay itsself is pretty standard action-RPGish combat). The DS of course doesn't have a built in light sensor and DS cards are far too small to have adaptors on them. So it'd have to go in the GBA port (which has also been called an expansion port). Which would probably mean that it'd need both DS and GBA carts in the box, which seems like it'd raise the price... oh well.