Hey guys, please spread the word, I found a fix! I have had no problems playing with wifi enabled. As a test I even played 2 entire circuits on Mario Kart 7 multiplayer with a friend. I also was able to browse the internt, and play my various other games. I was having the same problem, if wifi was on, I would get the black screen of death telling me to hold down the power button and restart.
Anyways, I opened my 3DS XL, found the wifi card, unplugged it, and plugged it back in.
Here is a picture of it http://img.gawkerassets.com/post/9/2012/...
Just be careful when unplugging it. You should grab on each side and just wiggle back and forth. DO NOT PRY IT OPEN like a leaver, as the connector is a tiny strip on the edge closest to the center of the 3DS. Here is a picture of it unplugged, so you can see what I mean about the tiny connector plug in spot, so BE CAREFUL. http://img.gawkerassets.com/post/9/2012/...
Opening the 3DS in this fashion will not void any warranty's, so don't worry, there was no warranty sticker that I had to tear through to get this far, thank god.
Also when I opened the 3DS, I noticed my "R" button was unplugged, so I plugged that back in as well. That was not what fixed it, because I immediately tried it after that, and still got the same error whenever I turned the wifi back on.
So, in short, just pop her open, unplug the wifi card, and plug it back in firmly till you hear it click. Mine clicked twice before it was all the way in, so make sure you push down firmly. And just like that, I was back in busyness with my 3DS with the wifi turned on!
For me this has been a permanent fix. If this is just a temp solution in the long run, then at least it will help you to do a system transfer to back up all the stuff you might have bought on the 3DS online store. That's actually my plan, I just bought this thing a week ago, so I am going to take it back and do a system transfer with Best Buy, just to be on the safe side.
I hope this helps!
*Edit Aug 7 2014*
For those of you having issues finding all the screws and opening it, here is a video showing how it is done.