I occasionally go through days or even weeks when I need to intently focus on a hard problem. I don't know what separates those periods from my usual work, but for years I've found that setting my normally bright and photo-centric wallpaper to something neutral, calm, and non-distracting helps get me into the right frame of mind. It doesn't help me focus, but it points me in the right direction to get started towards that state.
Anyway, for years, whenever I need that "boring" wallpaper, I have two standards I reach for:
- Tiger's default aqua wallpaper (scaled to fill)
- iOS 4's linen texture (tiled)