From E3:Boktai
From IGN

IGNpocket: Where did the idea for Boktai come from? Did you see the light sensor technology first and say "Wow, this would make a great game!" Or did the game idea come first?

Hideo Kojima: It wasn't technology first. I wanted to create a game that involved sunlight, and with the Game Boy Advance you can carry it outside, and there are no other games that involve sunlight. And I love the theme of Dracula and vampires, I've always wanted to come up with a game that players can fight vampires. It was a lot of different ideas that came together, and I thought, why don't I create a game that involves fighting vampires with sunlight? But I didn't know if it was technically possible, and I didn't know how much it would be if I could do this because it might be too expensive with the GBA ROM and having a specialized cartridge. There's a division within Konami that makes specialized toys that have odor sensors and humidity sensors, and they're pretty cheap. And when I was creating Metal Gear Solid 2, I asked them if it was possible to come up with a solar sensor. And when I knew that it was possible, that's when I said "We're going to do this."

More in link.

The light sensor converts sunlight into usable energy in the adventure, so the more time spent out in the sun, the more powerful your character can get. With this kind of inovation, there's no doubt that boktai will be one of the cooler games to hit the GBA. Gotta start saving up....