Konami had two new Metal Gear Solid titles to announce at this year's E3. One puts you back in control of Big Boss in a series of missions that will eventually lead to the rise of Outer Heaven in the 1970's. The other follows Raiden, though the timeline for this game hasn't been announced yet.
The PSP exclusive title, Metal Gear Solid: Peace Walker, is set just 10 years after MGS 3: Snake Eater and has Costa Rica asking Big Boss for help repelling an invading army. So his army without a government pairs up with the nation without a military. Since this game focuses on the "Soldiers Without Borders" becoming "Outer Heaven," Peace Walkers is being touted as the missing link in the MGS franchise, and will supposedly lay down the groundwork for the original Metal Gear game.
Not much information was handed out during E3 about this title. What we do know is that Hideo Kojima and the MGS 4 team are focusing heavily on this title. The rest, we can only make assumptions about based on the trailer that was shown. At one point, we see several Big Bosses on the screen; the most obvious possibility is some kind of co-op capabilities that will allow you and some friends (I believe 4 were shown at the same time in one scene) to work through a level together.
Unfortunately, we know even less about the other title, Metal Gear Solid: Rising which will be released for the Xbox 360, PS3 and PC. While Kojima has stated that both this game and Peace Walker are conical and official games for the MGS series, he will have less of a role in Rising so he can focus on Peace Walker. The only clue as to when this game takes place seems to be the fact that Raiden is seen on top of Crying Wolf in the trailer, which should place it before Metal Gear Solid 4: Guns of Patriots.
Neither of these titles have release dates, so we can't really say when to expect them, but I'm sure the two development teams won't make their fans wait too long before setting any kind of hard date.