In response to a challenge on the Thingiverse Blog http://blog.thingiverse.com/2012/02/21/oo-someone-make-one-of-these/ I offer this first attempt to print the visual illusion of balls rolling uphill. When viewed from the correct angle, this piece should give the illusion that all 4 ramps slope up to the middle platform. Viewing from any other angle gives the game away!
Print the part, then orient it so that you view it as seen in the rendered view, and it should look like all 4 ramps slope up to the central platform.
I haven't printed one yet, but I couldn't resist the challenge!