Interesting that all the replies are saying this never happened with a 5D mk 2 or other earlier body. I have owned my mk 2 for just over a year, and I have similar wear in an isolated patch on one of the bottom corners. It appeared rather suddenly a few months after I got the camera, and it does not seem to be getting much worse since I noticed it. I'm glad to have found this thread since I was wondering if this was normal.
I had assumed that I had somehow managed to jam the camera against a lens or something in my bag, although I'm always very careful when I put it in there. This 5D2 replaced a 20 year old EOS 10S and represents several times what I have ever paid for a camera before, so I tend to be rather protective of it.
I have around 4000 actuations on the camera, which gets used mostly for travel photos and at home. It is certainly not subjected to heavy professional use.
The references to BlackRapid might explain how this happened. Now that I think about it, the wear probably appeared a while after I bought a BlackRapid RS-4 (which I love by the way). So my best guess is the bottom corner of my 5D2 is brushing against my clothing as I walk or hike with it hanging by my hip. Its not really a big deal - my only concern is that I have been rather tempted to sell it and buy a mk 3, and it might have some impact on the resale value. But I've decided to stick with it for a while until I feel I have a better handle on digital technique after decades of shooting film. I want to be ready when I step up to a 5D3, 7D2, or whatever Canon has out there in the future.