Some days you've just got to do something daft. Today is one of those days. CSS wasn't supposed to do this (square peg, round hole), but hey apparently it can made to do it if you really want to :)
That's right, this is CSS 3D, not WebGL or Canvas, so see if you can figure out how I did it with lighting and irregular shapes. Then check out the source code if you like.
TweetUpdate:I thought it'd be fun to allow the light source to move.