Balazs Vagvolgyi has created an ultra-slick 3D Rubik's cube for the iPhone and iPod Touch.  The cube loads fast, looks great and plays smooth.  I really like this version because you can rotate the cube in three directions and each row individually as well. 

Balazs has created a great Rubik's cube here that will satisfy anyone looking to get lost in the cube.  Instructions pasted from the site below.