It's quite timely that just as Google warns of drive-by downloads, news has emerged on the wires of Didier Stevens' experiment.
Stevens, a security professional based in Belgium, launched a Google Adwords campaign entitled "Drive-By Download. Is your PC virus-free? Get it infected here!" six months ago. So far he has had the ad clicked on 409 times, although it's not clear how many clicks were by humans, and how many by bots.
Read Stevens' blog post here.