I managed to log a user in using OAuth. Then I can retrieve all the user's contacts by calling http://social.yahooapis.com/v1/user/me/contacts?format=json&count=max.
But I can't find a way to get the user's own email address. How can I get it?
I ended up using OpenID+OAuth in order to get the user's info with OpenID and the user's contacts with OAuth: http://developer.yahoo.com/oauth/guide/openid-oauth-guide.html