I just stumbled across this great article at the Institute for Lifecycle Environmental Assessment,
it talks about the lifecycle energy cost of disposable versus renewable coffee cups. This quantifies the energy needed to produce, use and reuse the products.... The website also has other great assessments. I am still going to continue to carry my reusable mug, but am going to make sure that I get its true worth!
Addendum (r.e. andrews comment): I am not in any way saying that we should use disposable cups or anything. One of the most interesting things I find about this article is how it separates out one aspect of the environmental impact of a product. There is no question that it does not take into account the environmental impact of billions of cups going into a landfill...but it does beg the question, if we could make a disposable cup that was 100% compostable (and actually was disposed of correctly) would then that tip the environmental impact towards pro disposable. I think mostly I posted this because like The Story of Stuff, it makes you think about the true cost and holistic view of an items environmental impact.