Although I need to spend more money on gaming-related goods about as much as I need another hole in my head, I can't help but crave the custom, Miyamoto-branded 3DS case below.
It was created by the proprietor of the KickassCases etsy shop, by the way, and it can be purchased (here) for just $20.
Looking for a custom 3DS case, but not interested in one that features the grinning visage of Shigeru Miyamoto? You still may want to check out the KickassCases shop--especially if you're a Mario, Yoshi or Zelda fan.
I'm pretty fond of this Kirby one and this Tetris one myself.