Massive Incorporated launched the world's first video game advertising network.

Massive Incorporated allows advertisers to simultaneously reach an aggregated audience of gamers through real-time delivery of advertising across an entire network of video games.
The Massive Network is supposed to deliver ads seamlessly into the gaming environment with no impact on game play or performance, making in-game advertising painless and unobtrusive for gamers, publishers, and advertisers alike.
Billboard ads in games are nothing new, so i guess the impact for the gamer is not really high. For advertisers this system is huge. They can now run campaigns as they would run them across web-sites. Additionally Games are more and more replacing TV programming. The advertisement market for the virtual reality will grow dramatically.

Massive has already signed exclusive in-game advertising agreements with Vivendi Universal Games, UbiSoft, and Legacy Interactive.
More details in Massive Incorporated Press-Release.