The nighttime mission is a thing of the past -- or at least, only half of the thing of the present -- in Metal Gear Solid: Ground Zeroes. It just wouldn't be an open world if there weren't days and nights, so guess what? There are days and nights.
Not that there were never any days in the Metal Gear series; both Snake Eater and Sons of Liberty were largely done by the light of day, as well as most of the spin-offs taking place largely under the sun's watch, but the in-game cycle is a novelty to the series. This isn't too weird, considering the fact that the whole open world thing is equally a novelty, so it seems like this is as good a place as any to input this.
This isn't just for the aesthetic, either. Kojima went on record with Eurogamer to tell them that the change in time period brings about other changes, such as troop patterns and other such, as of yet unrevealed, things to improve replayability. Given how many times I've played through Snake Eater myself, an improvement in replayability is really saying something.
The world isn't completely persistent, however. Traveling to new areas will cause loading screens, so don't expect to roam the countrysides without any interruption. Given the history with the series, this may be anything from a plain black screen to a scene card for the next area's name.
No word on release dates or consoles, but Kojima assures us that it will be ready to ride on modern consoles, so probably PS3 and Xbox 360... Just don't expect it to look like the the PAX footage.