On PvP servers Darkmoon Faire is a sanctuary zone in an overall contested zone.
Contrary to a sanctuary zone which comprises the entire zone, the Darkmoon Faire sanctuary behaves more like the Argent Tournament sanctuary in Icecrown.
What this means, is entering the zone will flag you for PvP. You will be unattackable (for now) but will be flagged.
The juicy part is what happens when you take the portal back to Elwynn Forest or Mulgore.. you are in friendly territory and still flagged for five minutes =)
This means people leaving the Darkmoon Faire can be ganked when they leave, this is a nice steady supply of world PvP victims.
50+ kills in Mulgore over 1 hour:
http://i40.tinypic.com/1z73j2b.jpg
Enjoy it while it lasts, there isn't any decent world pvp anywhere else in wow at the moment.
Edit:
Video on how this works and the results here:
Free Tips on WoW - Darkmoon Fair - YouTube