Great post by BL Ochman on What's Next Blog, called Doing a Viral Campaign Ain't Cheap, or Easy.
An important, and sometimes overlooked principle is that "a viral campaign requires not only social media, which provides remarkable new tools, but also integration with offline marketing, from street teams and guerilla marketing, to billboards, TV, radio, and print."
I would also argue that viral is not a channel that can necessarily be controlled. A true viral effort doesn't start out that way; it becomes viral by virtue of the audience connecting with it and making it viral. What clients (and sometimes we) term as 'viral' could otherwise simply be categorized as 'interactive.'
As I've said here before and constantly barrage by colleagues with, social media by itself is not a solution. It's another option in an expanding array of marketing channels. In the BtoB space, it may not always be appropriate for a client, but it's important to have a working knowledge of the range of new media.
It's nice to see my assertion backed up by BL Ochman: "New media marketing is simply not a substitute for all others. It is a tool: one of the best ever created."