First of all, I know that this technique isn't exactly "new". However, as I recall it, the old way was that your passengers could jump through walls by dismounting. This has in most cases been fixed. However this way is still fully operational.
Note: I posted this in the exploit section even though my videos name has "Exploration" in it, due to the fact you can use this method to exploit the terrain and gain an advantage in the game. (WSG comes to mind first, but also to skip bosses etc.)
This video shows skipping first boss in AQ40, some random jibberish in AQ20, easy way under Orgrimmar, and a example on how to get under Northrend fairly easy.
YouTube - Exploration using your mount
Obviously all you need is a mount, preferably a mammoth. I believe the mammoth is larger than other mounts (I dont mean by looks), and your character also sits a bit higher than on regular mounts. Then, find yourself a nice corner where the roof pretty much hits your head, then you should be able to mount up through it.