In this preview, Gamespy reports:
"The action starts roughly thirty minutes into the storyline of the game, and Snake's gotten some cues that Liquid Ocelot is encamped in a Middle Eastern country. It's Snake's job to make his way to a rendezvous point on the map while avoiding the AT Corporation's tanks, which are code-named "Irving." Snake calls them "lizards." Speculate as you will. He gets a beep on his codec, which loads up the new video chat-like Codec system. As in Snake Eater, you can fast-forward through the cutscene, or skip it entirely.
You'll still stand, crouch, and crawl with X, but there have been additional gameplay tweaks. For instance, the Square button now modifies your aiming. The game defaults to an auto-aim in the vein of the first MGS on PlayStation, or you can hit Square, and in a dose of irony, the camera reverts to the Splinter Cell-style over-the-shoulder camera. Triangle acts as a context-sensitive button for picking up enemies, interacting with the environment, and also for attaching to walls and blending into them. You'll still pick items with the secondary shoulder buttons, so your new oil barrel and trusty rations can still be found on the left side, while your armaments are on the right side..."