1) Find any referrer that is coming from a search engine (look at the referrer string to determine this). 2) Prune the search term that is a part of the querystring of the referrer. 3) Use this information to enhance your page's targetting on the fly.
Here's an example: Automatically isplay to the visitor in BIG BOLD HR tags, right near the html where the adsense ads are stored: "You searched for widgets. Widgets are highlighted on the page below. For more information on Widgets please read on..." or something similar. Just get the search term in 3 times (in a non-spammy way).
I'm not quite sure I understand how this achieves the desired results. Is it to draw the visitor's attention to the ads location or is it to get Google to display more relevant ads?
If it is the latter, I don't think Google will see the text since when the bot visits it will not see the searched terms the visitor used.