IGN DS has posted their preview of Mario Kart DS, which they were able to play at Nintendo of America's press summit, both locally and online. IGN's Craig Harris sums it up thusly: "In our short time with the game, it's very clear: Mario Kart DS is awesome. Awesome."
Harris went over details of the game, such as the new Blooper item that squirts ink all over the main screen, requiring you to use the map screen to navigate your kart, and the addition of drafting (tailing another kart to get a burst of speed). In his playtime, he found the second map screen to be useful when looking out for incoming shells.
The preview also notes that the Internet connection (press were able to play against other press members in Montreal and Germany) was "a bit temperamental"; lag reared its ugly head and there were a few random disconnections. However, this could be chalked up to the very high concentration of wireless sessions going on in the room, and is not necessarily an indictment of the Wi-Fi Connection itself.
Harris also noted a few changes that had to be made to the game for Wi-Fi play. Most notably, in Wi-Fi matches, banana peels and shells cannot be held behind your kart in order to protect yourself from an incoming attack.