When Seth Godin runs a viral marketing campaign, typically he's giving away value to build trust in hopes that it leads to some kind of sale (maybe you'll buy his books and tell everyone you know how big of a marketing Guru he truly is).
Another type of viral marketing campaign is kind of linked into the core product or service but serves more as an attention grabber - something to make you laugh, play with and send to all your other friends. We've seen campaigns like this when it comes to Burger King's Subservient Chicken, the movie The Secretary's little tush slap app and the following link that encourages people to learn Yiddish in a very unique and funny way.
As with everything, marketers are looking at ways to raise awareness and, more importantly, to get people to tell the people they know about something they've uncovered (that's long talk for "tell everyone you know!"). I'm not sure typing in a command for a guy dressed in a chicken outfit to act like Bruce Less is going to sell chicken burgers or if slapping a secretary's butt to hear her reaction is going to drag you into a movie theater, but if used properly - to create buzz, loyalty and value - there's no doubt that the budget to execute a key viral campaign will pale in comparison to the awareness, mass returns and business growth... if done properly. The Internet is also the best medium, at this point, to execute that kind of brand lift because it's simple to pass along by email, instant messager, Blog, etc... which is many easy touch-points within one dynamic environment.