In the demo I received from satyr (see oZone3D.Net forums), there is a toon shader that uses glsl uniforms. The pixel shader looked like to:

uniform float silhouetteThreshold;

void main()
{
  silhouetteThreshold = 0.32;     

  //... shader code
  //... shader code
  //... shader code
}

This pixel shader compiles well on nVidia gc but generes an error on ati. The error is right since an uniform is a read-only variable. This is an example of the nVidia glsl compiler laxity. That’s why I code my shader on ati: if the code is good for ati, we can be sure it will be good for nvidia too (of course there are always some exceptions…)