The Trick
Without Cold weather flying, you can go anywhere in Northend, as long as you don't dismount.
Reason
Some players prefer to scout the area before dismounting, because of those pesky gankers and base raiders on opposite faction.
What you need
*Being able to fly in Outland, or having Druid Flight form.
*For non Druids, you need a fly mount.
How's it done
As Alliance: Mount up anywhere in Eastern Kingdom or Kalimdor and take the boat that goes to Northend. Once you hop on, wait until you go to Northend and once you are there, tap the jump button to float for a bit. At this point you have a few choices: Fly around, Dismount, or wait for boat to stop, when I testes this I waited for boat to stop.
For Horde: Well you see, it doesn't matter the faction, as long as you can get a boat ride to Northend or a Zeppelin, and as long as you are mounted, you should be good.
STAY mounted to benefit!
Bonus:
If you have a fly mount or you are a Druid, mount up or go to flight form and enter Darkmoon Faire Portal, you will be able to fly around, but if you dismount, lets just say you lost your wings.
Tested where
Molten Wow (Cataclysm 4.3.4)