That's a different beast. Graphics drivers don't require a reboot because starting with Vista it is possible to restart them. You can try that for yourself: go to device manager and disable the video driver. Notice how the screen goes blank (and you might
see a text mode cursor flashing) and after a while the desktop reappears but using the generic software driver. And of course, you can re-enable it and get back to normal hardware accelerated mode.
As for hot patching: not all updates support hot patching and it's enough only one update that doesn't support hot patching for a reboot to be required. And I'm not even sure it applies to client version of Windows, all information I've seen on hot patching
talks about server versions.
Pretty sure that nVidia still asks for a reboot. ATI does not anymore, only during uninstallation.