I think the argument for imposing a tax on the carbon content of imports arises if other countries are not imposing the same carbon tax. If the whole world got together and harmonized their carbon tax systems, then you would choose on a worldwide basis to do it either on a destination basis or a source basis. But given that you're doing it unilaterally, the reason for imposing a tax on the carbon content of imports is precisely to keep an even playing field for your industries with respect to other countries.
On April 7th, 2008. See this statement in context.