The Official Nintendo Magazine in the UK has an interesting paragraph on Mario Kart Wii. According to the magazine, MKWii will feature Battle Mode as an online option with up to 16 players and you can also now pull off stunts mid air. A bunch of the old MKDS tracks are also going to be recreated for the Wii version along with a few brand new ones. Mario Kart Wii is set for a Q1 release next year, but considering the trend in Australia, that could mean whenever they feel like it..
Source: GoNintendo