"Since most people use cell phones these days, we decided text messaging would be an easy, convenient addition to our password recovery options. To set up password recovery via your mobile phone, just sign in to your account and click Change Password Recovery Options. Enter your mobile phone number and current password and then click Save. If you lose access to your account for any reason, you'll be able to regain access by entering a code we'll send in a text message."
For now, the options is only available in the US, so you need to use a US proxy to see it. Google also updated the password recovery settings page to include all the account-recovery options: secondary email addresses, text messages and the security question.
Update: the feature is now available everywhere.
{ via Blogoscoped Forum }