Rich,
With 12,000+ workstations, at my last place of work, we monitored the networks for all kinds of things. Pictures would come streaming by and at the push of a button, tell me the IP address, user, and all kinds of information and give me a listing of all URL's within the past 6 months that the user had visited, along with all pictures in thumbnail size, clickable to expand. All is in the name of security (and possible policy violations) Big Brother CAN be watching.
But of course, you had to have access to the network firewall to do this.