LAN mode (must be connected to a local network or the internet) At the main menu press L2+R2+Triangle
http://www.gamefaqs.com/ps2/914941-nasc ... 004/cheats
We tested it out recently on Xlink Kai and discovered that only 2 people can race together at once. Also, the game supports USB KB chat. We didn't test out usb headset support yet though.