Found in 2.56a and r35537 on Ubuntu 10.04 x32 and r35757 on XP32, using Geforce 6800GT

If you are in Multitexture or GLSL mode and "textured solid" is active, the hardness slider of a material that is assigned to a curve affects the highlight of other objects with completely different independent materials.
This only happens with curves that have a bevel depth >0.
Meanwhile the hardness setting of non-curve objects only affects the viewport if "textured solid" is deactivated.
If background image is checked (no image required) and unchecked after the hardness has been changed, unchecking it will change the specular on affected objects.

Changing the window draw method affects the maximum brightness of the effect slightly but is not able to eliminate it.

In the attached file, just slide the hardness value of the "sphere" material of the curve up and down and you will see. The hardness value of the sphere-object on layer 2 does not affect the highlight of the cube although it has the same

material as the curve.