If you haven’t noticed, KDE SC 4.5 comes with a new wallpaper named Stripes. It has replaced the old default_blue that has been our friend since 3.x days (and maybe even earlier, I don’t know).
Since it is used in KDM, it can’t be (yet) shipped with multiple resolutions, but due to its simplicity it scales rather well.
If you are not satisfied with the default resolution (strangely enough it is 1440x900), you can download the multiresolution package from my gallery at deviantArt. Unfortunately, dA doesn’t accept .tar.gz so the wallpapers are zipped.
EDIT: Note that this is not the same version as the one shipped with KDE SC - this one (apart from multiple resolutions) includes a gaussian-noise hack Nuno suggested to make gradients look better.