It is easy to interface the Google data API by using the library Google  supply, for .NET too. Let’s have an example of acceding the contacts information:

This class requires the following references:

• Google.GData.Client
• Google.GData.Contacts
• Google.GData.Extensions

All these are available in precompiled form after installing the Google Data API setup. Of course the complete API contains method to interact with a loot of good things in addition:

• Blogger
• Calendar
• Calendar Resource
• Code Search
• Contacts
• Content API for Shopping
• Documents List
• Email Audit
• Email Settings
• Google Analytics
• Google Apps Provisioning
• Google Health
• Google Webmaster Tools
• Notebook
• Picasa Web Albums
• Spreadsheets
• YouTube

The only missing point: there is not (yet) a version for WP7, and the current codebase is not easy to port. Another missing point is that the API does not support OAuth2, that is indeed supported by the Google platform itself.