Google has introduced an advanced opt-in security feature called 2-step verification that makes your Google Account more secure by helping to verify that you’re the real owner of your account.
2-step verification requires two independent factors for authentication, much like you might see on your banking website: your password, plus a code obtained using your phone. Over the next few days, you’ll see a new link on your Account Settings page.
Getting started with 2-step verification: http://www.google.com/support/accounts/bin/static.py?page=guide.cs&guide=1056283&topic=1056284
