From Wei on the oauth list:
We are happy to announce that the Google Contacts Data API now supports OAuth. This is our first step towards OAuth enabling all Google Data APIs. Please note that this is an alpha release and we may make changes to the protocol before the official release.
See announcement thread for endpoint URL details, supporting tools and implentation discussion.
For more on the Contacts API, see the developer’s guide (although bear in mind it may not be up to date w.r.t. oauth).