Mine behaves as Fanoush says. Display shuts off, but WiFi keeps on going.
It's kind of weird walking around work with a SSH server running in my pocket all day.

Anyway, if it is able to sense the cover's magnet to turn off the light, then it is aware of the cover being on. So surely we can 'listen to the DBUS'.
I'm a programmer, but I'm a little busy preparing for an overseas trip, so I haven't got time to mess with this at the moment. Besides, I am a complete Linux noob.

But thanks to Stefan Tomanek's explanation, I am sure someone can figure out how to tap into the dbus message relating to the cover, or maybe a script that it is already calling. At that point, disabling wlan or just turning on 'flight mode' (didn't know it had one) would suffice.