This information will be useful for anyone looking to replace their original Nintendo DS unit with the Nintendo DS Lite in the future.
According to the FAQ on the Nintendo WiFi site:
Q: What happens when I share Wi-Fi games with a friend for use on another Nintendo DS?
A: The Nintendo WFC ID and the game card are treated as a unit. Because of this, playing your Nintendo DS game card in another Nintendo DS system may override the online information saved to the game card. This would include changing the Nintendo WFC ID associated to the game card as well as your own personal Friend Code. Other players with this Friend Code would no longer be able to identify you as an online friend until you provided them with the new Friend Code you would need to generate. However, the Friend Roster will remain intact. If you are planning on going online with your Nintendo WFC-compatible DS game, we recommend against using the game card in anyone else’s Nintendo DS system.
This means that your Nintendo WiFi identity is tied to your DS hardware.
Penny Arcade’s Tycho has discovered a way to transfer your Nintendo DS identity from one DS unit to another:
There are things about the way online connectivity works on the DS that are, for lack of a better word, “eccentric.”
At its root are a) Friend Codes you exchange, and b) the unique hardware ID I mentioned previously. Every online capable cartridge has a menu item that puts you into their “Wi-Fi Connection Setup,” which was focus tested until you could drop a penny on the touch screen and get online. In the options menu, the bottom item actually broadcasts a program to another DS using the “DS Download Play” feature I’ve discussed before, only instead of a game it’s sending a program to facilitate the transfer of your network configuration. I found this crafty.
Tycho doesn’t go into further details, but I’m sure that someone will publish a step-by-step guide for this sort of thing once the DS Lite is in more hands. I’ll update this post when that information emerges.
EDIT: Apparently, it really is that easy. Nice!