For reference to anyone else with the same problem:

The following works on NVIDIA but not on "ATI Radeon HD 4800 Series"

In the application:
Code :
	glDisable(GL_CLIP_DISTANCE0);
	glDisable(GL_CLIP_DISTANCE1);
	<-- Note Disable clip!

In the GLSL vertex shader:
Code :
	gl_ClipDistance[0] = -1000;"
	gl_ClipDistance[1] = -1000;"

The gl_ClipDistance is used, although disabled in the application via glDisable(GL_CLIP_DISTANCEi)