I recently had a discussion with a friend about a similar situation -- incentivized downloads in the App Store to reach high rankings. There is a continuum of advertising tactics, from directly buying downloads, to buying placement inside apps that give some benefit for installing an app, to advertising an app in as many places as possible where a lot of users would see it.

They all start with the same input (money) and end up with the same result (more apps installed, higher chart position), just they go about it via different means. Where should the line be drawn?