Google's AdSense program is designed to provide relevant targeted advertising content to web pages. It works, sometimes intermittently, on weblogs - but there's often enough other non-relevant things on the design of those pages to work only indifferently. What you do find however is that the appropriately targeted wiki design - especially one where the subject matter expertise of a professional association is involved, and where the words in use a specific to a field of specialty - yields extremely targeted results.
Caution is worthwhile, however, because for every well-targeted advertising stripe you get the occasional horrible, unprofessional blunder. Some cases in point.
Quimica from the American Chemical Society is running this ad on page one, hosted on Wikispaces:
Now ask yourself, what is to blame here? There are three ways that you can fix a problem like this of having inappropriate advertising running in your wiki.
1. Don't run advertising at all. Host your wiki yourself, or host it somewhere with an option to turn off advertising. Remove the risk of the accidental inappropriate Adwords ad by not using Adwords as an ad network.
2. Sell advertising in-house as sponsorships, or run house ads in the stripe of the page normally designed for external advertising. This might very reasonably be ads for things like upcoming association events, membership benefits, or other details that look like advertising targeted straight at your members.
3. Use the Google AdSense ad quality as a measure of the relevance of the pages you have, and edit the wiki until the junk ads don't show up. Block advertisers that are inappropriate, remove page text that triggers inappropriate ads, and otherwise optimize for relevance, professionalism, and revenue. This works best when you are hosting a wiki on a platform and get the revenues for the advertising - some "free" wiki hosting really is paid for by click-throughs on ads that you are running, and if your association members are using Google Adwords to promote themselves they may well be running ads that would be very relevant on your site.
4. Use Google AdSense quality as a quality check on the professionalism of the AdWords advertising that your association members are running. From time to time, advertising copy writers at agencies will write ads that are completely inappropriate because of regulation or professional standards that they are blissfully unaware of, and by having a spot which attracts these ads you can build in feedback for professional ethics.