The iGoogle developer sandbox has always served as the bleeding edge version of iGoogle. It's the place to go when you want to be the first to try out new features. Unfortunately, if a bug sneaks into a sandbox release it can grind gadget development to a halt. This puts gadgets.* and OpenSocial developers in a tough spot, because the sandbox is the only place to develop with these features. Or, at least, it was.
Following on orkut's model, iGoogle will now have a "production" sandbox, meant to provide a more stable development environment. New features and improvements will hit the regular sandbox, first. After they've had some time to simmer, they'll move over to the production sandbox.
To enter the production sandbox, first enter the regular sandbox, then append the 'force_prod=1' parameter to your iGoogle URL. If you are not already in the sandbox, 'force_prod=1' will not trigger the proper behavior.
If you're hitting the REST/RPC endpoints, you should now use http://www-opensocial-sandbox.googleusercontent.com/api and http://www-opensocial-sandbox.googleusercontent.com/api/rpc for the sandbox endpoint, and http://www-opensocial.googleusercontent.com/api and http://www-opensocial.googleusercontent.com/api/rpc for the production equivalents.
As always, feel free to discuss sandbox issues in the developer forum.