According to Official Nintendo Mag UK, snaking has been removed from Mario Kart Wii.

The second big difference (in Mario Kart Wii compared to the rest of the series) is the removal of snaking. Ever since the N64 game, practically every Mario Kart game has included the ability to get a speed boost by waggling the controls left and right as you powerslide. This has finally been scrapped, and now your speed boost is determined by how long you can hold the slide.