Got the sound working, that sounds good. Maybe I can make an NES emulator with a Wii nunchuk. Hmm, I wonder what the NES protocol is? I could have a second Arduino reading the nunchuk and outputting the appropriate serial stream.
Hmm, a bit of Google-fu seems to indicate that the emulator needs to respond to a /LATCH and then respond with 8 bits upon receiving a clock signal. An SPI slave configuration might achieve that quite nicely.
Oh well, that might be tomorrow's project. 