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Google has finally released their open source container sample as promised today. After looking through it and the accompanying documentation, it definitely provides a good starting point for potential OpenSocial developers to begin forming the basis for their applications.
The API specification is considered version .5 and unstable, but provides the various XML DTDs, JavaScript interfaces and other data structrures required for a service to make itself compatible with the OpenSocial platform. Google intends to add real security, application isolation, and other "helpful things" once the development kit is officially released.