Wii Speak Channel. Unfortunately, the code only works once, locking secondhand purchases out of the dedicated chat-to-your-friends channel. According to MTV Multiplayer, the channel will not be available through any other means, and Nintendo will not replace lost or stolen codes.
If you were keen on conversing with your Wii pals outside of supported games (i.e. Animal Crossing: City Folk), you'll either have to buy new as of Nov. 16th, or invest in one of those telephone things. On the bright side, telephone numbers are even easier to remember than friend codes.
[Via Nintendo Wii Fanboy]