Sure, Starbucks celebrated Tax Day by offering a free coffee to anyone who brought in their own reusable mug or go-cup. But did you know they always offer a 10-cent discount for customers who bring their own vessels?
Their goal is to serve 25% of the beverages made in their stores in reusable serveware or tumblers by 2015. Judging by the number of people I see in my building stopping at the lobby's Starbucks and bringing paper/plastic cups directly upstairs to their desks, that seems rather optimistic, but good luck with that!
Anyway, save the planet, save a dime. Then donate that dime to Greenpeace.