System Information
OS X 10.10.1
ATI Radeon HD 4850 512 MB

Blender Version
Broken: 2.76 48f7dd6
Worked: unknown

Short description of error
Scripted driver expressions can be used to animate objects in various ways, but they do not work for material node values.

For example, we can make an object rotate back and forth using the expression: #cos(frame*pi/30)
but we can't use such an expression to make a pulsating glow on a material by using that same expression on an Emission Shader's strength.

To use a simpler example, we can use the single-variable expression #frame to increment some value each frame. It doesn't really matter what the expression is - driver expressions aren't working in material nodes.

They are working in compositing nodes, from what I have observed.

Exact steps for others to reproduce the error

1. Create an object and a material for it.
2. In one of the material's nodes (for example an Emission Shader's strength) type in a scripted expression as the value. ("#frame" will do)
3. Play the animation and see that it doesn't work.

Here is a .blend where the object is animating just fine, but the material is not animating even though the same expression is being used. There is no change in the render, and the node's value does not change as animation is played. The emission strength value remains at -0.5 no matter what expression is inserted. (Before saving the file it showed as the default 1, but after saving and re-loading it shows as -0.5)