Here are some press articles covering Word of Mouth Marketing:

Who do people trust - Forrester

WoM Marketing Spending to Break 1 Billion in 2007 - PQ Media

Did you hear that - MediaEdge

Where's Debbie - MediaEdge

Disclosure does not hamper WoM Marketing - AdAge

WoM the most powerful selling tool - Nielsen

The Next Big Thing, or Just a Buzz? - Keller Chadwick

Marketers Discover WoM Benefits - On the Mark