There are a bunch of systems that do this. It is not really part of the wireless systems at all. Many hotels that have wired systems also feed you though this logon screen. It is a firewall or something similar that redirects the traffic to a local machine and then dynamically allows you to have internet access after you authenticate. Many times they also filter the sites you can go to or prevent certain types of traffic. There are many many ways to do this.
One thing to remember is even if a user does not authenticate he still is on the wireless network. This means they can attempt to attack other machines who have also connected to this system so be sure you do not have your firewall disabled and such.
You can find lots of information about this if you search for public hotspot software