The developers for Tomb Raider have stated that there's not going to be an optional demo, so that your experience won't be "spoiled." What's the next logical step? To release 11 minutes of pure gameplay online for all to see of course!
In this latest video, the "Monastery Escape" demonstration showcases some of the stealth and combat elements, as well as an example of the game's gruesome deaths. Eidos' Creative Director walks us through the video, and highlights mechanics like Tomb Raider's adaptive AI and platforming changes.
You're going to have to wait for the full release to see multiplayer, as this walkthrough is entirely a single-player affair. Of course, if you're looking to come into the game completely dark, I wouldn't watch this.