There have been a number of interesting articles in the news recently about the national security implications of virtual worlds and cyberwarfare. Start your reading with a couple of Danger Room articles, one on "Virtual World For Future Army Training" and an earlier article on "Army Wants 'First-Person Thinker' Video Game".
Next, stop over at Intelfusion for Jeff Carr's thoughts on cyberwarfare and the game of "Go". I have long thought that you can inform your understanding of a particular culture's strategic thinking by understanding the games people in that culture play. Go is a particularly good example of this, in my estimation, and Jeff's association of the game with cyberwarfare helped me think about the cyberwarfare issue in new ways.
Finally, if you want to understand where the future of virtual worlds might be, take a look at this chart on the KZERO website (KZERO is a research firm that covers virtual worlds). It doesn't take long to see that the millennial generation is growing up with virtual worlds and will likely drive their development. Finally, for an example of an existing virtual world targeted to this generation take a look at Club Penguin or, if you are more interested in the next generation of virtual worlds targeted at the millennials, see Gizmodo's coverage of Lego World.