All the update does is essentially fix a few exploits found in the game, for instance the Wuhu Island/Mountain Loop and Bowser's Castle 1 shortcuts. A number of people around the web are hilariously reporting that potential cheaters are already attempting to load up Wuhu, and are miserably failing when trying to use the old shortcuts.
Thank goodness for Nintendo's willingness to embrace patches! Before the DSi, people would have been left in the wind. Hundreds of people on various Mario Kart community message boards were refusing to play online with these glitches intact, so chalk up a definite win for Nintendo.
Mario Kart 7 Update Available Now [Nintendo Life]