EDIT2: Hey guys. Unfortunately for me, my screen just died. Fortunately for you, I had to put my old wifi card back for warranty. So, I made some pics and a manual. Enjoy!
Download disassembly manual (1.32Mb)
Also, I'm sorry this took so long. Honestly, I was afraid to open it again myself. But it's a piece of cake!
EDIT: Apparently it's easy!
Once you've got all your screws, HDD's, ODD and SD-card all out (be sure it's all out), you can pry open the laptop at the front! Put a flathead screwdriver between the aluminium and plastic of the cover and work your way along the front of the laptop.
Then watch out with all the ribbons that are attached to both the top and bottom of the laptop.
Your wifi-card is located somewhere under the E, R or T keys of your keyboard.
So it's quite easy, you just have to dare it.
I feel you, there aren't any guides yet. Tried it myself.
Undid all screws (hidden ones too), even got four plastic hooks loose (under the battery), but the cover wouldn't bodge. Even e-mailed Asus (in the Netherlands, but they wouldn't help me, I think because they don't know either :P). I'm gonna try the international Asus office, or their facebook page.
Let's hope someone figures it out soon, I want my wifi card exchanged! ;-)