Mario Kart Wii will be out in the US on April 27!!!!!!!!!!!!! Why so many exclamation points? Feast thine eyes on this:
This game has been announced as supporting the Nintendo Wi-Fi Connection service. This will feature online racing and battle modes, both of which are capable of up to 12 simultaneous players. It has also been confirmed that there will be online leagues, with international and local rankings. This will take place from within an entirely separate Wii Channel. This channel will also feature the option of sending saved time-trial ghost data.
IGN has several videos for your online viewing pleasure.