They're designed to be compact, remember this was designed with the Nintendo DS in mind, not the wii which is homeuse. Also Nintendo design peripherals with children in mind so their products have to be fairly robust.
They may use microswitches similar to their joypads (I hope), worst case they just make another squishy keyboard. I don't mind the diamond arrow key setup, as long as its not the same layout Macally MK96 layout which was truely horible or the D-pad like Microsoft Arc Keyboard arrow keys.