The REST api for opensocial makes its appearance in opensocial 0.8. The specification references some other specifications that are also worth a look.
OAuth Consumer Request – This is proposed as the means for server to server authentication between the Consumer site and the Service provider. It has the potential to replace basic auth over SSL which is the only real standards based approach for securely authenticating using a shared secret, given that digest was underspecified.
XRDS-Simple – This also looks promising and it is tackling the whole xri / yadis discovery mess that openid 2.0 seems burdened with.