I did read an article recently where Kojima said MGS3 would be the last.. he said it completed the Snake storyline and wrapped up loose ends, etc. That it was a trilogy, and only a trilogy.
While it might not be the last Metal Gear game, it'll be the last Metal Gear Solid game. I suppose any future games, especially without Kojima, would just be "the continuing adventures of Solid Snake" type of things. And most likely without the "Solid", since Kojima seems to want to take that with him.