Every HN reader knows that. But "normal" web users do not know it (anymore?). Every so often I see some anecdotal evidence of that when I watch people doing searches.
There are also limits on disguising search results as ads.
>For the rest, there's not much short of an acknowledgement overlay that will get the message across reliably (for text ads -- image ads are easier, but google doesn't have them)
Really? Just increasing the contrast or adding a border around the border or using extra spacing as a separator or another separator can help a lot. But it's hard to make changes that hurt the bottom line. Changing it the opposite way is much easier. Google has a lot of UI and UX experts and I don't think the changes are accidental at all. In fact, they must be very carefully planned.
Edit: Found an old screenshot where the difference is much more apparent.
In terms of industry norms, Google is about as good as it gets. Maybe you feel that's not enough, and I guess that's a qualitative judgment that you're free to make. People will differ on things like this occasionally :P. I'd ask for some kind of harm analysis at the very least before I accept that there's anything untoward going on.