Kind of plain but all in all nice work. I would add some more detail work into the green background. Maybe just images poking threw you know?
I have to agree, it is a nice basic concept however the top 2/3 is too plain. I would maybe add just some basic shapes (wouldn't need to many, even one well placed) in probably the brown colour you used (=
Oh man, is the bottom part brown? I'm legit red-green colorblind and I get dark green and brown confused sometimes :S
I just took a second look, I would say it was a very dark olivey kind of green, however the shadow of the cat looks brown in comparison to the light green background. As an idea of how to break up the top third, you could just draw a narrow line horizontally across the page in that third colour. To make it slightly more interesting though you could get it to follow the outline of the cat when it passes over that region. Does that make sense? I did not know how to explain it.
I like it. It is a wallpaper so I think its better that its plain. If your like me and have heaps of icons, i wouldnt want the wallpaper to have too much going on. Also i think most people have most of their icons to the left of there screen. It might be better to switch the cat to the other side so that its not covered by Icons.