A new teaser image has appeared on the Kojima Productions website. A couple of green exclamation marks equals what looks like the Xbox 360 power symbol (with an exclamation mark). All in green with the text "Next Metal Gear is...". You can check out the teaser here... it looks like Snake is turning green. Stay tuned.
(All the screens are from Metal Gear Solid 4: Guns of the Patriots on PS3)