@cshaffer: you are talking about ClearPass Guest 3.9 (AmigoPod). With ClearPass 6.x the Palo Alto integration has been removed from ClearPass Guest and has been moved to ClearPass Policy Manager (CPPM)
Within CPPM you can add an Endpoint Context Server, which can either be a Palo Alto firewall or Panorama. CPPM will be able to update the IP-to-User-mappings in the Palo Alto using the Palo Alto's XMLAPI. CPPM will have to receive accounting information in order for this to work.
CPPM will only send the username to the Palo Alto; but it is not anymore restricted to the guest application, this will also work for 802.1X.
This integration can be very useful if you need to traffic and need to know which user has sent that traffic. You can also use the UserID information in your security policies.