A couple of recent interesting articles on using Starbucks as an office, prompted at least in part by their shift to providing free WiFi.
GigaOm's Using Starbucks as Your Office? Here are Some Tips does as the title says and provides a list of tips for those working at Starbucks.
It also references the Xconomy story Should it be Called "Startbucks?", which provides a great, short description on the advantages of working from Starbucks:
"To an entrepreneur, having tables, chairs, free Internet, and access to food and coffee is a golden opportunity. To a professional who provides support services to startups (e.g. marketing, design, business planning, business development), the local Starbucks just became a sales office. To a recent graduate or a talented programmer looking for her next big opportunity, Starbucks just became a productive hang out."
One of the habits I've gotten into is counting the number of people who appear to be working whenever I visit a Starbucks. This very anecdotal count consistently runs in the 30% - 40% range.