... And they completely miss the point. Yes, you can hire a firm to do word of mouth marketing (also known as Roach Marketing) where you hire people (or even real raving fans) who are compensated (money, product, etc...) to "spread the gospel."
Does it work? It can.
Burger King's Subservient Chicken spread virally and through word of mouth because of what it was (funny, new and questionable), not who it was from (Burger King. Did you know? Do you care?). We now know that it did create brand lift, heightened awareness and increased sales for their Tendercrisp Chicken. Ultimately, if you have to pay a person to "lie" to someone else, I'm not sure that's word of mouth marketing.
Word of mouth marketing starts by creating a product or service that is worth mentioning (read Seth Godin's book, Purple Cow). From rock bands (KISS) to technology (Google, eBay) to specific coolness like iPod, Blackberry and Hotmail - a great word of mouth marketing campaign is based on one core element: the product or service kicks butt and people will rant and tell all of their friends about it because it helps simplify their life, makes them feel good or even for bragging rights.
Word of mouth marketing works big-time - but only if you build that type of uniqueness into the product or service as it is being created. It rarely works after the fact.
Take a look at the Montreal Gazette article (it was not online), and let me know if you think they hit the mark or simply featured many companies who you can pay to create non-sustainable buzz and market share.