The IP of the Client is also noted in the volatile environment of the View Agent at the time a client connects\user logs in. This is located in the registry at "HKCU\Volatile environment\ViewClient_Broker_Remote_IP_Address". Currently I am using BGInfo pulling from this location so that we can have our users provide us the Client machines IP address and machine name. With this same thing in mind you could run a logon\logoff script to send this info to something as simple as a text file or even a SQL database. Also, this area provides much more info as well making it a good resource and easy access to a lot of informaiton.
Good Luck!
