[swiley]

GNU/Linux doesn't do anything like that by default unless you where to write a script to.

I honestly don't see a good reason to do it although one of my friends pointed out that using TOR is probably one of the better ways to check for this since it won't rely on a specific server.

[icebraining]

Well, GNU/Linux itself is not an OS, but Ubuntu and Fedora do enable that check in NetworkManager by default, whereas Debian doesn't (for privacy reasons).

[zaarn]

Linux doesn't include a desktop or browser by default.

Firefox will make these requests as does Google Chrome and Chromium. NetworkManager will do them too and various GUI tools for network configuration will try it.

Otherwise Captive Portals would simply break your internet without notice.