If you've ever used one of these institutional wireless offerings, you know how they work: first you connect to the Wi-Fi network, then you pop open a browser tab and try to navigate to any external site (Apple, TUAW, CNN -- I know someone who does a Google search for 'hello' as his connectivity probe). If you're not authenticated yet, the service throws up an interrogation page asking for your credentials, or just an acceptance of the terms and conditions -- click through and you're online. Most airport & hotel Wi-Fi networks behave this way, as does AT&T's WLAN in Starbucks stores, and the subscriber networks from Optimum Online, Time Warner Cable, Boingo and others.
In Lion, you can skip step two. The OS itself notices that you're not connected all the way to the Internet, and puts up a small Webkit window where you can log in or accept the T&Cs. Handy, and helpful!
iOS users will notice that this is yet another 'import' from Apple's mobile system, as the iPhone and iPad already work in exactly the same way.