I apologize if this is repeated somewhere in the forums, but I couldn't find mention of it myself. The URL for the Google Checkout Sandbox has changed:
http://groups.google.com/group/googl...id=-QVE1zTmcuw

So if you are already using the Zen Cart Google payment module (in sandbox mode for testing), you have to go into your configuration table, find the Google Checkout entries (probably towards the last rows) and edit the fields accordingly.

If you haven't run the install script yet, you can edit the sandbox urls in googlecheckout.php, so that the correct values get stored in the database.

Hope this helps someone, I was momentarily mystified.