Making scales stick to the surface (skin) of a model is difficult. But this is the simple part, making them behave like scale is where you hit a brick wall. Some would claim it is stupidity to even try. I tried.
2 years later
A working rig.

With 22 scales.

The scales closely follows the surface when it flexes.

Even at extreme poses the scales stays neatly on track
This is by no means a perfect rig. The scales are anchored to surface and can only rotate in the axis parallel to the root side of the scale. This means they tend to tilt up when twisted in certain directions.

There is still a long way until any usable animations can be made with this rig:
http://www.youtube.com/watch?v=guqlPff7fks
Edit: Woops, wrong forum, was supposed to post this in the artworks section.