By ethically acceptable, I mean that it is completely disclosed to all developers and users of Drupal, and that it is not mandatory (users can disable it if they want). No sneaky business like the .css trick used by Wordpress, or a link farm, ...etc.
I am not sure what they exactly mean by "integrate AdSense support into Drupal". Do they just want us to make the Drupal default page for the admin to has a link to signup for Adsense? A standard block for Adsense?
As long as it is fully disclosed and can be disabled, I do not see anything wrong with a voluntary referral program, since this is effectively what it is.
Nobody thought about it that Drupal would use any non-transparent methods to introduce this, or at least I would never suspect that, from how much I know the guys and girls here. Goba