Not just you. There is a slight Curve to the back from side to side. Not sure why, but from a design perspective, this curve will allow the device to lay flat on a surface with less rocking that a perfectly flat device would. Unless both surfaces are perfectly flat, the Device would rock. Making the Surface slightly concave would improve it's ability to lay flat without rocking. Ideally three points of contact are the best, but that would look odd on a rectangular device. Again the design comments are just my speculation as a design engineer. Wonder if they would look at me weird if I stop into the Microsoft store with a straight edge and start checking the display units. I bet the sales associated don't realize the back isn't perfectly flat.
It's very slight. I can see it when I sight down the edge, but it's not clear. When I place a straight edge across the back I can see light under the middle. Again this is slight, but a pretty consistent arch that doesn't look like a defect to me. I'll need to check a few other to see how consistent it is. Maybe I'll stop at the Microsoft store once the mall isn't crazy.
Mine is definitely slightly concave. Again it's subtle. Can't really measure it accurately, unless I took it to work and use a granite table and run an indicator across, but it's there. Now I more intrigued to check some others to see if the manufacturing method for the case of the Surface is consistent or not.
I'm very happy to hear, that's it's not just me. I see the exact same as can be seen in Sin's picture. It really is very subtle, but it's there and I just wanted to make sure that my new Surface isn't shortly before cracking into two peaces
Happy New Year,