On a Nokia 5800 you get virtual navigation keys from the device that enable you to do D-Pad input as soon as you start a Gameboy game with the emulator. For the joypad buttons A, B, Select and Enter you can go to the emulator settings and activate on-screen buttons, so that you can use the touchscreen to press these buttons. You can do the same for the D-Pad, but IMHO this makes only limited sense since the device gives you larger buttons for this anyway. Additionally you can use the Nokia's orientation sensor to control the D-Pad. IMO this makes sense only for a few games. So there should be enough input options also on a Nokia 5800.