When deactivating the specularity of a material node setup, the specular disappear(this is correct) and the shading for this material becomes a lot darker(this does'nt seams to be correct, at least, this does not occur in 2.49b).

A blend file is provided with the minimal setup. All you have to do is to uncheck the specular shader in the node setup. When rendered it is correct, but in the viewport, the shading is darker.

You may need to rotate the viewport for the shading to update.