Let's begin with a definition of "functional foods" provided by the Institute of Food Technologists:

foods and food components that provide a health benefit beyond basic nutrition (for the intended population). Examples may include conventional foods; fortified, enriched or enhanced foods; and dietary supplements. These substances provide essential nutrients often beyond quantities necessary for normal maintenance, growth, and development, and/or other biologically active components that impart health benefits or desirable physiological effects.

For those researching functional foods (and functional beverages too) here are some good resources. For an in depth discussion of functional foods, including health claims, check out Functional Foods: Opportunities and Challenges. Another excellent resource, especially on claims for emerging science, is Guidelines for Communicating the Emerging Science of Dietary Components for Health published jointly by IFT and the International Food Information Council Foundation. Finally, the Nutraceutical & Functional Food 2006 Buyer's Guide includes product and contact information for nearly 300 companies supplying ingredients within 23 categories.