Random Shapes

1. Start with a black layer >Filter>Noise>Add Noise.
For more control, you could "paint" a shape with a diffused brush.

2. Blur the image with >Filter>Blur>Gaussian Blur.

3. Trim the Levels (CTRL+L) until you get a distinct black and white image.

4. At this point you can select the black or white area and erase parts of another layer.