I found some nice tutorials on the internet to use Google Authenticator with linux. Since I am a fan of full featured copy/paste tutorials that work I created my own. You can find it here:

Use Google Authenticator to login to a Linux host

Thanks to the following articles:

Nuno Goncalves
Use Google Authenticator to login to a Linux PC

Keith Garner
Google Authenticator (and implementing it on Linux)

Jens Roesen
Zwei-Faktor-Authentifizierung mit Google Authenticator und des kleinen Mannes Nucular Code Sealed Authenticator System