As all the communications and entertainment options converge and evolve, virtual worlds are going to become more and more prevalent, immersive and individually important.
If its up to marketers I think we all know what sort of world will evolve:
Of course the most immersive worlds today cost money. So can world providers market while you pay for access to their world. The consumers will decide. I would bet people will put up with quite a bit as long as its up front and clear from the beginning. World of Warcraft is one of the best examples of subscription world that eat people up. I almost think its too late in their social contract to do product placements of any sort. They need to leverage their pull externally from a marketing perspective and not comprimise anything in game.
As there are more and more people on this planet and people are more and more aware how small they are in it the need to virtual world will only increase. We are just starting to feel this phenomenon.