Hideo Kojima, creator of Metal Gear Solid, has stated his belief that Metal Gear Solid: Rising "will create new users." While speaking on twitter, Kojima said "the next MGS... "RISING." this game will create new users, be developed by the new generation, a new MGS." This seems in line with the small bits of information we know about Rising, namely that it will be more of an action-adventure experience.
On the topic of the upcoming PSP release, Metal Gear Solid: Peace Walker, Kojima was a little less clear, calling it a "new but classical MGS." In typical Kojima fashion, he's already speculating about a new Peace Walker before the current project is out the door. "PW is a new type of game design and experimental for future endeavors. So it is hard to say it will finish with one game," he said.
Peace Walker is scheduled for a May 25th release in North America on PSP. Metal Gear Solid: Rising has been announced for PS3, and Xbox 360, a release date is not known.