Blender 2.53 Beta - Material Diffuse Color Drivers can't control RGB parameters

I started to adapt my Pack Shot Studio project (seen in BAM 27) for Blender 2.5x versions.

Everything works well except one : It’s impossible to control RGB parameters with drivers of a Material Diffuse Color. By the past (before 2.5x versions) it was possible …

Perhaps I did something wrong or it may be a bug (to repport) ???

Here is a Blender file to illustrate the problem : diff-color-driver_problem.blend