By my definition, a viral campaign has to ride along on
borrowed interest. Viral marketing is more than getting a referral. It happens when a
marketing message gets a free ride on an otherwise unrelated
activity. The new Proctor & Gamble campaign for Crest with Scope
Extreme is one of the strongest viral campaigns we’ve seen recently.
Targeting the segment of adults involved in meeting new people to date,
the company printed ads on cocktail napkins, distributed them in bars
and encouraged people to use their cell phones to play a text-message-based
game called Irresistibility IQ.

Cincinnati Business Courier Procter uses phones to sell Crest in bars, 2006-Jun-26, by Lisa Biank Fasig

"It’s not super serious, You can see how a group of people in a bar would try this together," said Carsten Boers, president of Flytxt Inc.
Flytxt is among several firms Procter selected last year to help with a mobile marketing strategy, he said. Such marketing is common in Europe, largely because it is very cost-effective. Though he declined to specify the value of his contract with Procter, Boers said in general mobile campaigns run in the tens of thousands of dollars, plus the cost for products, such as cocktail napkins.