The old relationship API has a function called crm_get_relationships() that allowed you to grab a contact's relationships. However, it requires a contact object as input, and v2 API only deals in contact arrays (civicrm_contact_get, civicrm_contact_search).
So, here's a new version, civicrm_get_relationships() that you can stick in the v2/Relationship.php API file. I haven't tested much at all, but the code is pretty simple so it should work as before.
You can find the code here: http://forum.civicrm.org/index.php?action=dlattach;topic=3794.0;attach=428
Forum post here: http://forum.civicrm.org/index.php/topic,3794.msg16726.html#msg16726