Holiday Bromance

*** UPDATED FOR CATACLYSM (yes, I'm back) ***

How to find the Brothers (Alliance only - If you're horde you should be here):

You get Mistletoe by doing a /kiss on one of the Winter Revelers at inns (reports saying that in Dalaran it works only from the Revelers in Underbelly). You might get Fresh Holly or A Handful of Snowflakes instead though. (Original comment from Ongra)

The achievement wants to you use the Mistletoe on the brothers. Simply target the brothers mentioned below and then right click on your Mistletoe.

1. 
   
2. Start at Goldshire. Brother Wilhelm (41.0, 65.9) is standing at the back entrance of the Blacksmith
3. 
   
4. Ride or fly to Stormwind's Cathedral
5. 
   
6. Brother Kristoff (55.2, 54.5) stands next to the fountain statue at the Cathedral Square
7. 
   
8. Get inside the Cathedral
9. 
   
10. Brother Benjamin (52.1, 47.6) is walking up and down behind the right pillars
11. 
    
12. Brother Joshua (49.6, 45.1) is standing at the end of the corridor next to the High Priestess
13. 
    
14. Brother Cassius (53.2, 45.1) is inside the right room
15. 
    
16. Brother Crowley (52.5, 43.5) is at the entrance of the RIGHT catacombs. That's next to the entrance of the room Brother Cassius was.
17. 
    
18. Take the Gryphon to Rebel Camp, Stranglethorn Vale
19. 
    
20. Brother Nimetz (47.2, 11.1) is at the west side of the Rebel camp
21. 
    
22. If the Goblins don't hate you, take the Gryphon to Booty Bay, Stranglethorn. Then take the boat to Ratchet, The Barrens. From there take the Gryphon to Theramore, Dustwallow Marsh
23. 
    
24. If you are Hated with the Goblins, take a gryphon to the Menethil Harbor and from there the boat will take you right to Theramore, Dustwallow Marsh. Time-wise is about the same, it's about the route you want to follow.
25. 
    
26. Brother Karman (67.3, 47.4) is supervising the Practicing Guards (Theramore Combat Dummies) - he's right next to the entrance of the north-east building (Foothold Citadel)
27. 
    
28. Brother Anton is no longer required for the achievement.
29. 
    

From there you can either HS back to your home town, or take the Gryphon to Auberdine and then the Ship to Stormwind.

Enjoy :)

EDIT #1: Added coords for coords add-on users etc (Thanks Eldariel & Datundra)
EDIT #2: Fixed coordinates to match new SW map (Thanks Spinkler)
EDIT #3: Added the way to get the Mistletoe (Thanks Ongra)
EDIT #4: Fixed location of working Revelers in Dalaran (Thanks Diska)
EDIT #5: Updated for Cataclysm (Thanks all!)

Famous quote "Bros before Ho's" , meaning someone will choose his friends over a woman

An update for Mist.

If you are above 85 and theremaore is destroyed for you there is still a way to get to Brother Karman.

Located at 55,49 at the end of the bridge coming out of the west of Theramore is a member of the Broze Dragon Flight named Zidormi. if you speak to her she will temporarily phase theramore back to the was it was for you.