Two chocolate chip cookies are yummy. Eighteen chocolate chip cookies will make you sick. There's a similar challenge with defining sustainability. Adobe is considered by many to be a sustainable building material, so is rammed earth. But would these materials be sustainable at the scale and density of Manhattan? That's a lot of dirt. Sustainability is complex. Natural is generally good, toxicity is always bad, renewable is good, carrying capacity / quantity is important, socially equitable is critical, and on and on. How can you assure something can be repeated in perpetuity?
But, like LEED, having a rating system can certainly nudge things in the right direction, allowing consumers to make the best (or less bad) choice. Architect William McDonough and Chemist Michael Braungart introduced the concept of "Cradle to Cradle" certification in their 2002 book by the same title. They have since developed a Cradle to Cradle Certification System:
"Cradle to Cradle Certification provides a company with a means to tangibly, credibly measure achievement in environmentally-intelligent design and helps customers purchase and specify products that are pursuing a broader definition of quality.
"This means using environmentally safe and healthy materials; design for material reutilization, such as recycling or composting; the use of renewable energy and energy efficiency; efficient use of water, and maximum water quality associated with production; and instituting strategies for social responsibility."
The system has yet to rate many consumer products, but it seems to be moving toward the 'sustainability index' Janne is looking for. It will probably never be possible to give a pass/fail grade to a product stating definitively that it is sustainable (it can be produced/used/reused forever, at any rate/scale). Maybe that's the problem. The concept "product", and our consumer-driven society itself, are not sustainable. We're stuck within our culture's non-sustainable mindset, trying to make it less non-sustainable. As McDonough would say, we're trying to drive more slowly over the cliff rather than turning the car...