I too am having trouble with the Import contacts API. I am able to get all the way to an access token, but once there, when I try to use the given xoauth_yahoo_guid in the "http://social.yahooapis.com/v1/user/{0}/contacts" url, I get unauthorized errors. I have tried adding in the oauth tokens, both the ones from the request token and from the access token, but I still get an unauthorized error. Here is what my code looks like for building the url, and I use the basic OAuthBase class that was made for C#. Note, building it this way worked for every other step in the OAuth sequence. This is in a method that took an AccessToken class called token:
OAuthBase oauth = new OAuthBase();
Uri uri = new Uri(string.Format("http://social.yahooapis.com/v1/user/{0}/contacts", token.xoauth_yahoo_guid));
string nonce = oauth.GenerateNonce();
string timeStamp = oauth.GenerateTimeStamp();
string sig = ConsumerSecret + "%26" + token.oauth_token_secret;
StringBuilder sbAccessToken = new StringBuilder(uri.ToString());
sbAccessToken.AppendFormat("?oauth_consumer_key={0}&", ConsumerKey);
sbAccessToken.AppendFormat("oauth_signature_method={0}&",
"PLAINTEXT"); //HMAC-SHA1
sbAccessToken.AppendFormat("oauth_signature={0}&", sig);
sbAccessToken.AppendFormat("oauth_timestamp={0}&", timeStamp);
sbAccessToken.AppendFormat("oauth_version={0}&", "1.0");
sbAccessToken.AppendFormat("oauth_token={0}&", token.oauth_token);
sbAccessToken.AppendFormat("oauth_nonce={0}&", nonce);
sbAccessToken.AppendFormat("oauth_session_handle={0}", token.oauth_session_handle);
