Since the Atom Publishing Protocol specification was finalized, we have been working on making the Google Data APIs compliant with the AtomPub standard. As of today, we are releasing a new version of most of our services that achieves full compliancy with RFC 5032.
If you're worried this this may break your current application, rest easy. This change will only be available in API version 2 and higher. If you are happy with the current version, you can keep doing what you've been doing and the API will continue to work as it always has. If instead you'd like to use the AtomPub compliant version (and the new v2 features), just specify API version 2 in an HTTP header, and you're good to go.
One of the new v2 features is the use of HTTP ETags for optimistic concurrency. ETags are a web standard that work well with HTTP caching. The client libraries that support version 2 will handle ETags automatically, but if you are interested in how ETags look at the protocol level, check out the details in the Google Data Protocol Documentation. Some services are using V2 to introduce additional improvements as well (for example, YouTube's new geo-search feature), so be sure to check out the documentation for your favorite service to see what's new.
For those of you who'd like to try out version 2 in our client libraries, the Java and .NET client libraries have been updated with V2 support. To see a list of the services who are V2 compliant and to find out how to migrate your apps to the new version, check out our migration guide. We recommend migrating to v2 if you can, as any future improvements will be introduced to version 2 and higher. For further information about the release, please check out the new Google Data documentation or head over to our discussion group.
[NFGB] Link - from Official Google Data APIs Blog