Sounds like a very interesting future for indie game developers, IMO: Google Adsense for Video Games
. They are planning this for flash games first, but want to move this to real (sic) games later. Just like anyone with a small webpage can earn money using adsense now, maybe any hobbyist or professional game developer can do this with their games too, soon.
The bad part is how censorship via the in-game adverts will affect creativity, I don't think any major corporations would sponsor a dark, dystopian future where those very corporations are the bad guys.