UPDATE: HOW TO NOW AVAILABLE http://www.instructables.com/id/Make-Your-Own-Game-Boy-Android-Gamepad/
I wanted to play Nintendo emulators on my phone, but did not like the touch screen input. I've heard this complaint time and again, all over the 'net. I've seen some great looking commercially produced solutions, but I wanted something more. I wanted to enhance the experience by actually playing on classic Nintendo hardware, and at the same time not having to plug in via USB. Not to mention I love making things.
Using a Wii remote as the Bluetooth interface, and a Dot Matrix Gameboy's butttons and shell (two things I already owned) I figured "why not mash them together and give them a whole new purpose?". With the use of the "Wii Controller IME" App on the Play App store, this retro controller will really control anything which lets you use the android touch keyboard as input. Although i've only tried with a SNES emulator, in theory you should be able to play and do all kinds of things with the Gameboy's buttons. The project is essentially a gutted Game Boy with a Wii remote inside. The tricky part was rerouting the button connections between the two.
This project was built for the Galaxy Nexus, but can easily be altered to any Android phone or tablet. The limitation is really the support of your device with the Wiimote App. Technically you could control your wii with the Gameboy minus some functionallity.