I've tried finding a tutorial without any luck. Even if someone could describe the effect so I could better search for tutorials myself.
I am trying to achieve the following effect as seen in the aqua background of this image by using GIMP:
This tutorial should give you the tools to make those kinds of backgrounds. You might have to play around a bit in order to make your own unique one, though.
You can generally find tutorials about this kind of thing by searching for abstract Mac OS X wallpaper art.
The first time I started seeing art like this was in the blue Mac OSX wallpaper under "abstract". The design you showed above can be created by using the same techniques used to create those abstract backgrounds.