Ad age has quick blurb about companies and viral marketing. Their article indicates its mostly the under 25 crowd now that are the target, I think that is a little untrue, I know many people much older than 25 that are into You Tube, and Google Videos.

You Tube
Viral Marketing can take many forms but the emergence of user generated video has enhanced viral marketing with many viewers watching what they think are real videos only to find out later it was actually put out by a company like Mcdonalds or Smirnoff. Either Way they all seem to generate buzz, interest and traffic to the target website or business. Viral Marketing works.
Smirnoff Viral  Blog Marketing

The Advetising Age Article is Here..