NCSoft has announced that they are going to start with optional in-game advertising in City of Heroes/Villains, a s detailed here and here. Seeing these ads are optional and the player can select to not choose to see those ads.
Motivation for this is to get another revenue stream to cover the costs for the increased effort put into the game, instead of charging the players more in some way.
While I cannot say that I am particularly excited about advertising in the game, I can understand their position. It is a competitive market and with the current business models it can certainly be price sensitive. Anarchy Online has done it for years now, but City of is a bit larger title and it will be interesting to see how this may impact the player base.
We will probably see more of these changes in the future. I think this may be one potential driver for developing games with modern settings – it will be easier to introduce advertising in the game that does not seem to be out of context. You cannot really have that in a fantasy setting, unless the setting was something similar to Discworld perhaps.