Installed Google Authenticator module
Installed Mobile Codes module
Configured a code for my account, scanned the code into GAuthenticator.
Went to permissions, unchecked box "Login without code" for all groups. Saved.
Logged in with correct username and password only. Login worked (I don't think it should, unless I'm mistaken).
Logging in with correct username and password and a bad code. Results in a form error for the code box.
Logging in with correct username, password and code. Login worked.