Markus used an MMA7455L 3-axis accelerometer to detect the direction of the gravity in this cool demo:
I am using the MMA7455L breakout board to control a 3D cube displayed on a (SPI-modified Seeedstudio) OLED display.
My MMA7455L library is available in the forum.
Get the MMA7455L breakout board for $15 at Seeed Studio. Back in stock and shipping now.
Thanks Markus! Via the comments.