‘To increase monetization on your site and improve the relevance of ads, AdSense now varies the number of text ads that appear in a given ad unit. In cases where we determine that increasing the size of the most relevant ads will improve performance, we’ll drop the lowest-performing ad or ads and expand the remaining ones to fill the entire unit. Showing fewer ads works to your advantage, allowing the better-performing ads to draw more user attention and click-throughs. Google AdSense technology will automatically determine the optimal number of ads to display on any page and will only show fewer ads when doing so will make you more money!’
In other Adsense related news – Google are also making changes to Adwords (the Advertisers side of Adsense) and will test longer descriptions on ads (up to 200 characters). Read more about this at Search Engine Journal.