I've found that ever since I changed to FancyURLs (a feature my blog software has that allows me to change my post URLs from "http://obli.net/index.php?itemid=122" to "http://obli.net/item/122") Google seems to be liking the site more and more, my posts are indexed more quickly, and the ads are actually on-topic more often now (as opposed to serving ads from French banks when I'm talking about something completely different).

So here's my tip for webmasters with AdSense; try to use as simple URLs as possible, I have a suspicion that Google dislikes question marks in the URL.

Naturally, this tip is as any other tip about AdSense purely speculations, Google is very secretive about how their technology works.