Paul Krugman, an economist with no scientific background, blogs:

Testimony on the issue from a real trade lawyer (thanks to Richard Baldwin, who pointed me to this document). The bottom line:
In sum, if carefully calibrated along the lines suggested above, carbon equalization measures at the border, imposed on certain imports, can be modeled in compliance with WTO non-discrimination rules and/or the WTO’s environmental exception.
So the economics are right; it’s WTO-legal; and it would neutralize a major political argument against controlling greenhouse gases. Why, oh, why, would Obama say “Ni”?

No so fast, writes Planet Gore contributor Julian Morris in the “Comments” section:

Unfortunately, it’s not that simple. While ‘carbon equalization measures’ might be WTO legal (depending on how they are structured), the danger that they become a new form of protectionism is very real. One of the key issues is whether it is possible to calculate a realistic embodied price of carbon that reflects the implicit price paid by US producers. There are two issues here: first, if the US implements a ‘cap and trade’ scheme, the price will vary according to supply and demand, e.g.: if the number of permits is reduced, the price will rise; if new technologies are developed that reduce the costs of lowering carbon emissions, the price will fall. So, logically, the border measure would vary in accordance with the prevailing price. That will be difficult to implement. Second, the technologies used to manufacture and import any imported good, including the technologies used in shipping, will in most cases not be known to the authorities in the US who are responsible for setting the price, so they will not know how much carbon is emitted for each unit of produce imported. Which makes it rather difficult (i.e. impossible) for those officials to set tariffs at a rate that reflects the cost of the embodied carbon.
The result: tariffs will be set according to the demands of vested interests — including the energy intensive industries — with the likely result that they will be excessive and hence protectionist.