From the Starbucks website:
On April 22, we’re inviting everyone to bring in a reusable mug or tumbler and get a free brewed coffee or tea. Your choice of brew, hot or iced – it is spring after all! This is happening at participating STARBUCKS in the U.S. and Canada.
Why are we doing this? We recognize the impact one can make through careful, conscious choices. So we want to encourage and reward those of you who use tumblers or travel mugs in your daily routines. You’re helping to reduce our environmental impact!
Although reusable serveware and tumbler use still only accounts for a small percentage of total beverages served, those of you who did bring in reusable mugs still made a significant impact. In 2010, people brought their own tumblers into our stores more than 32 million times, keeping nearly 1.45 million pounds of paper out of landfills.