If you go into full screen mode while on the title screen the panorama background that shows some scenery is just a black to grey gradient. I've only tested this in the snapshot 12w49a but it could happen with other versions such as version 1.4.5. Im also not sure if this is intended but I highly doubt it. also one of the pictures I attached is with a texture pack and one is without.
How to duplicate:
Open up minecraft and go in to full screen mode after the menu screen comes up.