Online Guerilla Marketing (OGM). Companies setting up fake forums IDs and infiltrating comunities to generate buzz for their products. Apparently social communication being mediated by hidden advertising dollars. We probably all suspect that it happens, but I've never run across reasonably solid proof of the practice before reading this Escapist article:
Jack is what is called an "Online Guerilla Marketer," or "OGM," and his name isn't Jack. He's agreed to speak to The Escapist on the condition that we not identify him. Like an undercover cop or secret agent, Jack's effectiveness at his job depends on his ability to remain anonymous. He'll often spend days, even weeks, infiltrating a community to earn the trust of its members before he strikes - inserting a recommendation in the right place, at the right time to generate interest in the products he represents.

A typical day for Jack starts with checking "to make sure I haven't been discovered," he says. "I check logs and IP pings, and I revisit forums and posts I recently made for comments. I return to the threads, chat rooms or other places and respond to posts I have already made. I then go through and talk like a normal poster would - commenting on other subjects, talking about politics, throwing in some wood into a flame war, etc. - pretty much just to fool anyone that might be suspicious of my activities to show them that I'm a 'real' person and not some corporate shill.
...
"It's evil," Jack says. "I'm evil, and I will make you buy this commercial item or visit some site using any means necessary. That's my job, and I have to admit, I'm pretty damn good at it."

Now, I hesitated in linking the article despite its obvious interest to people who hang out on game community web sites like this because it would be incredibly destructive for community members to start hurling around accusations of corporate spying and instigating witchunts against members who say somthing positive about a game. So please don't do that, you'll find yourself receiving swift treatment from moderators for personal attacks if you do. And be assured that moderators will do what we can to detect these sorts of people who in any event, from the information in the article, tend to target "generalist" game forums where they're more likely to be able to hawk a number of different products.

But unfortunately, it's a question which will always lurk in the back of your mind... "is that person being paid to say that?" Thank you situational commercial ethics, thanks a lot.