Exactly: a good example is how OS X currently works - everytime you connect to a new WiFi hotspot it fires a request to a static page on Apple's server in the background, to determine whether or not you have to sign in. If it detects unexpected content it assumes it's a sign-in page for a WiFi hotspot, and presents it modally.

Under this proposed system the OS can simply check for an appropriate HTTP response code, which makes life a little easier.