From Breakthrough's analysis:

Cost containment proposals raise the question of how much low carbon technologies really cost. If, as many environmental leaders assert, cutting carbon emissions deeply will not cost as much as it now appears, and if low carbon energy technologies will soon be cost competitive with conventional energy sources, then environmental leaders should not object to cost containment provisions in legislation like the CSA.

But that is not what is happening.

Environmental leaders are attacking the cost control mechanisms and asserting that the legislation would do little to reduce emissions. These actions speak louder than all the rhetoric of recent years about solar, wind, and other alternatives being cost competitive with current energy sources. The reality is that alternative energy technologies, in real deployed terms, remain vastly more expensive than conventional energy sources. This is the reason that environmental organizations oppose cost containment and why even environmental supporters of the legislation see it as an incremental step that will need to be amended (namely removing cost containment) in order to achieve deep reductions in U.S. carbon emissions.

Full analysis here:

http://thebreakthrough.org/blog/2008/05/will_the_climate_ ...

The Stamford study you refer to focuses its analysis on the energy sector, where additionality is, as it points out, a major problem. But forestry, particularly avoided deforestation, can be quite different. The best estimates suggest that without action to conserve forests, 85 percent of the world's tropical forests will disappear, releasing between 87 - 130 billion metric tons of carbon dioxide into the air. What that means is that almost any tropical forest saved is an additional tropical forest saved (though you might want to provide a 15 percent discount for forest conservation credits). Of course, there should be a priority on protecting the most threatened forests, generally those closest to the agricultural frontier, but protecting tropical forests almost by definition brings additionality in.

I co-authored an essay about overcoming other challenges to deforestation with Dorjee Sun here.