Hey,
I'm posting this because i didn't find it anywhere and maybe someone got an idea how to use this or get this thing useful.
Step by Step:
1. Get on the Zeppelin from Orgrimmar to Undercity,
don't mount on an
flying mount! (Probably works on any other zep/ship with loading screen)
2. Wait till the Zeppelin moves and shortly before it reaches the point where the loading screen opens you need to disconnect urself.
e.g. using Disc Macro:
- Fly over the mountains like u can see here:
http://img6.imagebanana.com/img/mmg8mz62/1.jpg
- Look down with the camera and when ur at about this position us the Disc Macro:
http://img6.imagebanana.com/img/dffgb0jg/3.jpg
3. Now login again. You'll notice it worked when
ur char is standing in the air. If ur trying to move arround u will get disconnected instantly.
(If it didn't work ur falling down - prepare to use Slow Fall or Levitate or Parachute)
4. To avoid getting disconnected when trying to move either
use an
ability like
Levitate or use ur
Heartstone.
U now can't move or jump anymore. Ur character can still turn around.
Only way to move away is to get on an two-sitter-mount.
I tried to use a flight-path but it said "The character is currently busy." (poorly translated)
If u try to enter a bg u wont get ported in there but getting a deserteur instantly.
If u want to move again u need to logout and login again.
If u quit WoW by Alt+F4 or disconnecting and then log in again ur still unmovable.
Maybe someone has any idea or want to play around with it to do something useful with this method. Please share then 
I'm playing on EU Servers, if anyone is interested in testing some other things while being unmovable contact me via PM.
Greetings,
Wambo // Morpheus