Massive's technology is different than the average static ad in a console game. Massive focuses on online PC games. In this way, they can insert/remove ads at will as opposed to the console games where ads are inserted when the game is made.
However, with all of the next-gen consoles being online enabled, I don't see any reason that in-game ads can't be updated when, for example, you log onto Xbox LIVE.
Currently, about $10 million is spent on in-game ads, a fraction of the $10 billion on TV ads. However, by 2008, in-game ads are expected to reach about $100 million in sales.
What is kind of freaky is that Massive knows if you've seen the ad or not. If you pull your character away from the ad before you see the entire ad, the advertiser does not get charged.
Get used to it folks. It's not going away.