I think you're just looking into this a little too closely. I honestly do not for the life of me see anything in the upper left corner of the screen, and the things on the right just look like an effect that was copy pasted to make a "roll" effect for the mist. Nothing special. :shrug:
The line is very clear to me. I would suggest your brightness is lower than mine or perhaps your contrast.
I don't have photoshop on this computer so I can't point it out particularly well, but to be more descriptive: there's an isolated part of fog, or what may perhaps be a cluster of stars in the sky, which looks as though it should be larger than it is and cuts off down a perfectly straight line into darkness. I noticed it as soon as I opened the picture.
Edit:
Here's a picture for you, zoomed in. As I said, I'd increase the brightness / contrast to make it show up more but I don't have photoshop on this computer. Here ya go.
http://i53.tinypic.com/2ypgojs.jpg
Also I would generally agree with the idea of a certain brush being used to create the mist effect, but the fact it was so obvious to me made me think otherwise.
Edit Edit:
I also just checked the different versions of the wallpaper, including the 1900x1200, and can confirm that the line is there on those as well.