Right now Brewfest is out. If you hang out inside the area around durotar youll get a buff called "Brewfest Enthusiast." Which increases your XP gain by 10%. If you log out you do not lose the buff. Soooo, If you were to logout till WotLK comes out you get a bonus 10% XP for your first 2 hours.
To get the buff, quote from wowhead comments:
You get this buff by going to the brew fest camp outside Ironforge(alliance) or Orgrimmar(horde). Vol'jin(horde) or High Tinker Mekkatorque (alliance) will give out a speach two times a day. The speech starts a few minutes before 6 AM or 6 PM your server time. After the speech is over (which takes about two minutes) you get this buff.
Also, this effect DOES NOT persist through death. So be careful =)