It might surprise some hardcore environmentalists that using paper cups, or just using more paper on anything else, might be more environment friendly than you might imagine. (The following facts might be considered as flamebait again but please read on with patient before you mod.)
Papers are not made from cutting wood in rain forest anymore (some furniture, on the other hand, still are). 95% of the raw materials in paper are coming from trees, and these trees are carefully planned to grow and harvest. Various "Tree Funds" were raised every 10-15 years for raising money in building such tree farms.
Unfortunately, these "Tree Funds" are very sensitive to market. When there are less demand in papers, these funds would diminish, and in turn less tree farms would be built. Less tree farms, less trees, less oxygen-producers, more carbon dioxide, more severe the green house effect and so forth.
Encouraging paper-saving would probably lead to more green house gases. The irony...