A-Caroling We Will Go

Gaudy Winter Veil Sweater
Use this item in Stormwind, Ironforge, The Exodar, and Darnassus..
It is possible to enter each of these cities without dying or aggroing any guards, save The Exodar which has guards posted at each of its entrances and where you cannot fly.

Edit: It is apparently possible to do The Exodar using the Side entrance. Thanks addy2k!

I just completed this achievement (2011 version), here is my take on it (Horde on a PvE server):

Firstly the [item=70923] is obtained from opening presents under the xmas tree in your factions capital city.
NB: You will not be able to open the presents until 4:59 (server time) on the 25th of December. I logged in earlier and had to wait :)

You need to visit 4 opposing faction cities, wear the [item=70923] and use it to sing whilst in the city limits.

You can do the cities in any order and there are various approaches to getting in to the cities, but here is the way I did it:

Important notes:
You have to be within the city limits of each city for the singing to count towards the achievement. You will get flagged PvP when you enter the opposing faction’s city.

Exodar
From Orgrimmar take the portal to Nordrissil in Mount Hyjal. Fly west to the Ruins of Auberdine in Darkshore. Fly North-West from the coast (aiming for Ruttheran Village).

NB: You will get fatigue as you cross the ocean, but with maximum flying mount speed it only reaches about a quarter of the bar. I think it should be possible with all flying mount speeds, but I haven’t tested it.

You need to get on the boat which docks on the west side of the village.

NB: You will not be flagged PvP even when you land on the boat. I landed on the back of the boat to the left of the captain, but some others recommend sitting on the very front.

The boat will take you to Azuremyst Isle. On arrival you will still not be flagged PvP. Jump off the left (port) side of the boat before it reaches to the docks to avoid the guards and swim to the shore. Head for the entrance to Exodar which is nearest the docks (Not the main entrance). Hug the mountain to the North and you should see it soon after getting out of the water. Enter the tunnel cautiously, as there can sometimes be a patrol of guards here. You will be flagged PvP as you go in to the tunnel! You only need to take a few steps in, then you can sing and hearth/portal out.

This is probably the hardest of all the cities (risk of being griefed) as you don’t have many choices about where to stand and how you approach the city. On the plus side, most people can’t be bothered to go here and grief you because it’s out of the way.

Darnassus
You can use the same route you did for Exodar (see above). On reaching Ruttheran Village fly up (way up) and on to the top of Teldrassil. Head straight for Darnassus (West end of the tree).
You have many places to chose from to land. I landed on the hills to the North east of the city.

NB: You will be flagged PvP when you enter the city limits, but you are unlikely to be griefed unless you chose to land in the centre of town.

Stormwind
I took the zeppelin from Orgrimmar to Grom’gol base camp in Stranglethorn. Then fly North and again you have a big choice about where to set down to do your singing. I landed to the South-West of the Mage Quarter actually outside the city walls. This still counted as in the city.

NB: Some alternative routes you could take are to take the portal from a mage trainer to the Blasted Lands and fly North-West or complete the Ironforge part first and fly south to Stormwind. There are many other routes also.

Ironforge
I completed Stormwind first, so approached Ironforge from the south. One of the many alternative routes is to take the zeppelin from Orgrimmar to Undercity and then fly south approaching Ironforge from the north. On reaching Ironforge, I looked for a way to stand on the mountain above but didn’t immediately find anywhere that flagged me as in the city.

NB: It has been said you can stand on the city gates, but I thought this a too obvious place to wait if you want to grief.

I flew in to the city gates, being sure to stay as high up the walls as possible so I don’t agro the ground based guards. Directly in front of you you will see a large statue and above it a small hole.

NB: This hole is where the Alliance flight path enters the city.

I flew to the hole and landed there. Did the achievement, then mounted and flew out of the city again.

Key
Griefed : http://en.wikipedia.org/wiki/Griefer

It looks like we get this achievement by using Gaudy Winter Veil Sweater, which will be the gift for 2011. So we won't be able to complete this achievement until Christmas Day.