From the blizzard forums:
"When you get into an area you are not supposed to be it teleports you out now."
Appernetly you get a buff called "No Man's Land", with "You are where you should not be...." as the buff description, and after it expires in around six seconds it teleported you back to a preset location just outside the zone.
Ouch.
Now I haven't actually checked to see if this is actually happening, anyone want to go into hyjal and confirm this?