Advertising in Video Games?

While it does not provide much in the way of specifics (though I'm sure there will be much more about this in the coming months), this Reuters article highlights Viacom's interest in advertising in video games.

I'm not a gamer and don't really know how much, if any, of this is already going on. Should we be expecting product placements (i.e. game characters drinking a Pepsi), billboards in game landscapes, trailers before the game loads (like we see on DVDs)? I imagine the possibilities are endless and the demographic is certainly appealing. Has anyone experienced ads in games yet?