Since the game's announcement, there's been a lot of speculation about the theme of "nowhere to hide." Kojima had already braced himself for the inevitable questions about the setting being in a desert, or space, or the moon... but, in reality, the theme doesn't really factor in the setting, but rather a significant change in the series' stealth based gameplay.
Kojima had said that the MGS4 teaser video he put together for E3 had hints at the new gameplay change hidden in it. The most important one was at it's beginning when the wall the Snake's hiding behind collapses and he's spotted by all the enemies. He spoke with us about wanting to create a more dynamic, realisitic environment for MGS4 If you think about it, the environments in MGS1-3 weren't really dynamic. You could always count on a certain hiding place being there all the time. Not so in MGS4. In the game, you won't just have to plan a strategy around enemy movement, but also the movement of objects in the environment.
This was best illustrated by the example of Snake hiding behind a truck (wonderfully acted out by Randy for Mr. Kojima's entertainment). In all of the previous games, he'd be safe. But what if an enemy were to get in the truck and start driving it away? Snake would either have to run along with it or risk being spotted. "Exactly! You got it!," remarked Kojima. That's the premise of MGS4's new breed of stealth play: You can't always count on there being a safe place to hide. Cover might get destroyed, cars will move, cranes will move crates -- it will add a whole new level of challenge and realism to the series.