According to this press release by Starbucks the company supports sustainable coffee farming with its purchasing power.

Starbucks Coffee Company announced today that the average price per pound it paid for all its coffee increased from US$1.28 in fiscal year 2005 to US$1.42 in fiscal year 2006. During the same period the industry average market price was $1.04 per pound. Additionally, Starbucks announced that the coffee the Company purchased through its independently verified coffee sourcing and purchasing guidelines… [equals] 53 percent of the total coffee purchased by the Company.

Paying extra for coffee that is both better in quality and produced more sustainably gives support to farmers that practice responsible business. The evaluation guidelines that the C.A.F.E. (Coffee and Farmer Equity) program requires of both farmers and retailers cover a broad range of criteria, such as:

1. Hiring Practices
2. Worker Conditions
3. Protecting Resources and Biodiversity
4. Financial Transparency

SCS Certified is a third party that manages the C.A.F.E. program.

Carving deeply into their own profit margin (paying up to 36% more for coffee) to support environmental sustainability as well as social responsibility is a beautiful thing to see, especially in a company as large as Starbucks. Their business model, unlike most other large food service chains, has been to provide a higher quality product rather than cut costs as much as possible.