You mention "in-game ads", people get shirty. Don't like the fact they're being sold stuff within something they already bought. But there's less ground for complaint with in-game ads bought for a good purpose.
The Transport Accident Commission of Victoria, a state in Australia's south-east, have signed a contract with Massive—the Microsoft-owned in-game advertising provider—courtesy of which they'll beam their creepy anti-speeding commercials into the homes of the TAC's target audience: young Australian males.
The ads will appear in Guitar Hero 5 but, more importantly, in Forza 3 as well.
The TAC are famous not just in Australia, but throughout the world, for their insistence on promoting road safety via graphic, violent TV commercials and billboards. So it'll be interesting to see how the ads go down in Forza, a game about... driving really, really fast.
Below is one of the TAC's more notable efforts, from a few years back. Be warned; it's a little rough.
Video games to get local road safety ads [The Australian]