Tonnes of Information On DS Wi-Fi
Source: portables.p-nintendo.com/articles/I-89-2.html

Puissance Nintendo had the chance to interview Jim Merrick, here is the summarised version of the interview, for the full interview, click on the link in the source above.

-As Nintendo has stated before, you won't have to pay to play their 1st party DS games online.-Nintendo's WCS will be free for 3rd party publishers to use. They have the option of charging if they desire. They will own their own network-infrastructure. Use of the matchmaking service will be free.-Everyone will have a kind of buddy list. You'll be able to add people you know in the "real world," of course, as well as people you only know online.
-There are a few parameters you can set to fine-tune who you play against online. You could choose to only play against people you know (on your friends list), or you could choose to only play with people in a certain area (European players only, for instance), or you could choose to only play against players of a certain skill level. Basically, Nintendo wants you to be able to choose however you want to compete online.
-Each DS user is uniquely identified in the system. Adding people to your buddy list will be kind of like giving someone your cell phone number. Everyone will get a unique "password" of sorts each time they want to send someone a friend invite.
-You can have a separate name or "tag" for each game you play online. Also, you'll be able to pick names that people have used already (if you want to use "Bob," you can, even if someone else used it already). This works because each DS will have a unique userID (which you will never see).-You'll be able to connect your account to nintendo.com (or nintendo-europe.com). Your stats, friends, etc. can be viewed online.
-The Wi-Fi Connection Service will be launching worldwide (a specific date still hasn't been mentioned; though we know it has to be before Mario Kart DS comes out in November).
-Nintendo took way too long to release Animal Crossing (GameCube) in Europe. Even though Animal Crossing DS will be released in Japan and North America before Europe; the release delay won't be as huge as the original's.
-Mario Kart DS will be the first game to use WFC, probably followed by Tony Hawk's American Wasteland and then Animal Crossing DS. There are a number of other games that you will use WFC that haven't been announced yet.