This is Jacob from the Google Checkout team.
I'd like to let you know that we just released Google Checkout module for Zen Cart. This module provides Level 2 implementation of Google Checkout with Zen Cart, enabling you to provide Google Checkout to buyers and process orders through your admin UI.
This project is hosted at http://code.google.com/p/google-checkout-zencart where you can find the latest release at any time.
GOOGLE HAS ABANDONED AND NO LONGER SUPPORTS THEIR GOOGLE CHECKOUT ADDON
As of July 2012 a community member has picked up where Google left off. Please report any issues/ideas to: https://github.com/mattclements/Goog...Zencart/issues
NEW DISCUSSION ON THE NEWER MODULE can be found at: http://www.zen-cart.com/showthread.php?199125-Google-Checkout