Thanks for this, I always appreciate a straight to the point and useful tutorial. I hadn't realized the bump map/soft light combo existed within GIMP prior to this.
I would like to save some time for the prospective reader that wants a tileable texture- one method is to use the following technique to make plasma tileable
http://www.trueelena.org/computers/h...a_in_gimp.html

Obvious to some perhaps, but worthy of mention...
The radial displace and iwarp distortions will need some bounds boxing and cloning/blending touch-ups to remain tileable.
Also color band operations such as levels balancing can sometimes interrupt a cross-seam gradient leaving visual artifacts.