Official Google Data APIs Blog: OAuth for Google Data APIs

Official Google Data APIs Blog: OAuth for Google Data APIs
OAuth Authentication for Web Applications - Account Authentication API - Google Code

You'll now be able to use standard OAuth libraries to write code that authenticates users to any of the Google Data APIs, such as Google Calendar Data API, Blogger Data API, Picasa Web Albums Data API, or Google Contacts Data API. This should reduce the amount of duplicate code that you need to write, and make it easier for you to write applications and tools that work with a variety of services from multiple providers.