Postlethwaite explains that the ads will be appropriate in content and placement (e.g. the side of container trucks), non-intrusive and non-interactive. The developers have absolute approval rights. "If it's not appropriate or it's distracting, it won't go in," he said. No personal data will be collected. "All they track is if an how long you look at the advertisements."
Since we've yet to see any in-game ads in the screenshots, we don't know how well developer Splash Damage will implement them. However, that Postlethwaite is being open about the process is always appreciated, and he keeps to his word and it does mean more substantial support (e.g. free content packs in the future), it doesn't sound like a bad trade-off to us. The first sign of the advertisements will be seen in the upcoming public beta.