Procter and Gamble, the supposedly staid, old school marketer, has since 2001 been using a 280,000-strong army of kids to stealth market products using good ol' word of mouth. The group, called Tremor, enlists kids ages 13-19 via the web, offers them a chance to win a free product, such as a DVD player, and asks them to complete an application to become part of the group. Once in the group, they are sent coupons and sample products and delicately asked to share them with their friends.

Tremor also does work for non-P&G clients, such as Shamrock Farms, which tapped Tremor to promote a new chocolate malt flavored drink. Shamrock test marketed the product in Tucson and Phoenix. Tucson received the usual marketing efforts, but Phoenix was laced with 2,100 Tremor members who received product info, coupons and stickers. Following a 23 week test, Phoenix achieved 18 percent higher sales and 21 percent coupon redemption. Sandy K. Kelly, marketing chief at Shamrock, says those are the best results the dairy has ever seen.

Raining on the parade is George Silverman, author of The Secrets of Word-of-Mouth Marketing and an Orangeburg, N.Y. consultant, who advises caution: "It's like playing with fire: It can be a positive force when harnessed for the good, but fires are very destructive when they are out of control. If word-of-mouth goes against you, you're sunk."