We're now experimenting with serving up a small AdSense banner for users with less than 200 reputation. This banner appears just under the title of any question.
Up to 90 percent of our traffic is from Google now. We feel that users arriving from Google search will be accustomed to seeing a small ad banner at the top of the page, so it shouldn't be a bother.
While we don't want to pummel anyone with ads, we do need to advertise responsibly to ensure we generate enough revenue to fund continued development of the site -- specifically, so I can pay Jarrod and Geoff. I'd love to be able to hire Geoff full time, and continue to increase Jarrod's meager salary so it actually resembles something approaching professional wages. This money does not go into buying gold-plated humvees and designer megayachts. Yet. It all goes directly toward feeding your fellow programmers!
Once you've earned 200 or more reputation:
The question ad banner will no longer appear.
The two sidebar ads will stay, however, they will switch from a contrast visual style to a blend visual style (for text ads).
We aren't planning to have more than 3 ads, and as always, we will never accept animated, Flash, or Silverlight ads. We think this is a good, responsible balance of advertising, and hopefully sufficient to underwrite the continued development and improvement of Stack Overflow!
That said, like everything we do, it's an experiment. We'll continue to tweak and adjust as necessary.