You can see the Pointer Lock API in action right now. Make sure you're running either Chrome 22 or the latest Firefox release, and then fire up Mozilla's Bananabread demo. (It's a simplified, WebGL-based first-person shooter that looks a little bit like Quake III.) The game should automatically go full-screen and request your permission to disable the mouse cursor. See below:
Once you click "Allow," the game should behave pretty much as you'd expect any FPS to do. All you have to do to snap out of full-screen mode and un-trap the mouse cursor is hit the Escape key. Pretty keen.
Oh, of course, we're not going to see the Doom 4 running in a browser. Capabilities like these are nevertheless adding a new dimension to browser-based PC games. Who needs Farmville and cheesy Facebook games when you can fire-up a high-adrenaline shooter right there in your browser, without special plug-ins or local installations?
The Pointer Lock API should enable more than just great browser games, too. Google says the functionality will be useful for applications like "scientific visualization, training, simulation, modeling, authoring packages, and more."