Black Hat has written an interesting post on PPC arbing.

One thing that instantly sprung to mind (after lol, what a clever idea) was the recently introduced google term related to landing pages. If you have to have a landing page then you can’t directly pass the click to yahoo.

This also seems to be similar practice to “bottom feeding” - the art of bidding low on a topic with slightly off (maybe misspelled) keywords, and creating an extremely well optimized landing page, the only clickable link (or, to keep inside the TOS, the most obvious clickable link) being adsense.