I missed this April Fools' Joke, but that's probably because I don't live in Japan. Sega's prank was the fake release of what that they called "Game Gear Goju." It's a pretty straightforward attack on the Nintendo DS. Sega's new GGG handheld has 50 screens instead of a measly 2 screens.
Check out TechJapan's post if you can't read Japanese, and check out the GGG directly from Sega if you can.

