John Gruber links to some non-busy alternatives to the wallpapers that come with iOS 4. They're nice, but I just took a picture with my finger over the camera lens and used the resulting solid black as my home screen background.
Previously I was using the blackboard one (the fourth option in the top row) and it was non-distracting enough, but now for some reason (maybe the folder icons) I feel the need for something even less distracting.
I don't have the iPhone 4 yet but I find the wallpaper feature one of the least exciting. Reading some blogs/forums and users act as if finally this feature makes the iPhone complete! And those who can't get it one their current iPhone are screaming for Jobs' head! I can do without this wallpaper and just deal with a solid background color.
I feel the same way. Let's hope Apple doesn't decide to give us animated wallpaper.