You've always had a choice. Either leave your screen unlocked when you stow it and run the risk of pocket-dialing the entirety of your contact list or lock the screen and endure the tedium of entering a PIN every time you pull it out.
The new Samsung Galaxy Nexus, however, knows when to unlock itself—just by looking at you.
The Nexus, currently being unveiled in Hong Kong, uses its 1.3MP front-facing camera to scan and recognize your face—via facial detection functionality built-in in the camera software—unlocking the phone when it senses your gaze. No PIN typing, no puzzle piece dragging, no bar sliding. Just look at the Nexus, it'll look back at you, and then hopefully grant you access—though it did just Fail-Whale during the on-stage demo.