Gamespot has a preview of Boktai, the GBA game designed, produced, or otherwise associated with Hideo Kojima. It's an action RPG with a unique twist: the cartridge contains a light sensor. Sunlight falling on the cartridge affects gameplay - which, incidentally, is a neat copy protection method.
"The game detects sunlight via a sensor on the game cartridge. When the game detects light, it changes to reflect that. For example, when you're in an outdoor environment in the game and are outside with sun hitting the sensor, your gun will slowly recharge its solar battery, or you can speed the process up by forcing a quick charge. When you're in indoor environments in the game and have sunlight shining on the sensor, shafts of light will appear through windows, skylights, or other holes in the structure, which will let you recharge your gun while indoors. Though, while being able to keep your weapon charged in indoor environments is nice, sunlight plays a vital role when attempting to defeat the immortals. When you engage in a battle with an immortal, after dragging his or her coffin outside, you'll need sunlight to help power the special weapons needed to win, which pretty much forces you to play at least part of the game outside. While you can make it through the dungeons with judicious use of the Gun Del Sol, you absolutely have to be in the sun for a battle with an immortal. At the moment, there doesn't seem to be a way to fool the sensor, as we discovered in our various attempts to trick it with different light sources. So far, the sensor reacts only to good old-fashioned sunlight, so stock up on that sunscreen."
Weird input devices are cool. Additional gameplay dimensions are cool too.