Purpose of the MarioKart DS ID microformat is to identify the friend ID of a Mario Kart DS gamer. Each player can have a list of friends identified by such an ID to play with via the Nintendo Wifi Connection.
Being a microformat it can be easily embedded into an XHTML page, by using a class attribute enclosing the text contaning the ID. Simple example:
More extensive example showing it in relation to the hCard mircoformat:
If you use firefox and the amazing greasemonkey extension you can use the mario-kart-ds-id.user.js (right click "Install User Script...") script to alert you when a page contains a mario-kart-ds-id.