For the total GIMP newbie (Like me) I've split this process up more clearly.
When I first looked at RobA's post I couldn't figure out most of the techniques, so I wrote this as I went along.
I can recomend the GIMP Help Index files as they helped me to do this from RobA's descriptions.
So here we go.
Open new file
Add a layer and colour it black (Ignore the background layer)
Use the lassoo (sp?) (and shift key for more than one) to define some blobs
Bucket fill white.
Set radii to 100ish and apply
Open a new layer below this one (Helps to make the top layer invisable)
Filter>Render>Clouds>Solid Noise (Crank the X and Y values to max)
Slide the grey arrow in the centre to the left a little (about 1.9) and let go. The dark areas will become greyer.
Make the white blobs layer visable again and make it the active layer.
In the layers dialog box is a mode option with a drop down menu, click on the multiply option.
This should put the noise where the white bits were.
Then merge the layers together. (Layer>Merge Down)
Fiddling with the black and white arrows adjusts the outlines of the shapes.
Then press OK
Press OK in the pop up box
This gives you a white outline on black background.
This should give you a black outline on a white background.
I hope this helps, I learned loads doing it.