Now that I've had a few hours with Mario Kart DS, I must say, it's a really good game. Even my girlfriend, not much of a gamer, thinks so. I'd say it's console-quality, and definitely a solid update to a time-tested franchise.
MK DS includes 16 new tracks, and a selection of 16 tracks sampled from past MK games. Each racer (the usual 8: Mario, Luigi, Peach, Toad, Yoshi, Bowser, Wario, and DK) has two cars to choose from, with differing stats.
Online play is smooth and easy to manage, and it allows random matchups via WFC without swapping friend codes. On a related side note, the DS apparently doesn't speak WPA2-PSK, which makes me very sad, since I'm never using WEP, and I'm certainly not going unencrypted just for WFC. I may have to invest in one of the Ninty USB WFC gadgets.