Sign and Digital Graphics magazine has publish a couple of articles in their May and June issues relating to green in signage. I always enjoy reading articles focusing on green in trade publications and getting a feel for what the writers feel the majority of readers are interested in.
The May issue has an article titled Reduce, Reuse and Recycle. This article has an interesting chart of materials that are said to be green. The June issue has a follow up article titled Lean, Mean and Oh So Green.