From the Google Developer Blog:
The [Google APIs Client Library for .NET] handles OAuth 2.0 integration, streaming uploads and downloads of media, and batching requests. For more than fifty Google APIs, it is the easiest way to get access for any Windows developer. Whether you are plugging Google Calendar into your .NET Framework-based application, translating text in a Windows Phone app or writing a PowerShell script to start Google Compute Engine instances, the Google APIs Client Library for .NET can save you tons of time.
The beta version of the Client Library is already used by a number of Windows Phone app, but with the first stable release now available, the number of apps using the library, and therefore providing Google service integration should increase.
While each of the respective ecosystem owners would prefer consumers to remain with the borders of their own ecosystems and apps, the reality is that the majority of consumers will span one or more ecosystems and/or use third party apps to access the services within those ecosystem. That's why cross-ecosystem support and API availability for any service is a hot topic, as has been well demonstrated in the Windows Phone world with the Microsoft YouTube app.
The provision of the Google APIs Client Library for .NET is not a magic bullet and does not (necessarily) mean that Google services will now appear on Windows Phone. Indeed, the APIs that it encompasses were already available to Windows developers through the top level ReST, HTTP, and JSON implementations. However, the Client Library does make it a lot easier for developers to add-in Google powered functionality and/or integration with Google services.
For example, a significant number of Windows Phone app include the ability to load content to and from Microsoft's OneDrive cloud storage service. These same apps could add an option to load content from Google's Drive service, thereby offering their users a choice of cloud storage service. Another example would be to use Google Account as an identity / authentication conduit, in the same way that some apps and services let you your Facebook account for user authentication.
More information is available in the dedicated Google APIs Client Library for .NET blog.