Yesterday Google announced a "quality change" to the AdWords ad delivery system. Danny Sullivan explained that the ads displayed on a search result page will differ in quantity based on the commercial nature of the keyword phrase. So if a keyword is more educational (non-commercial) in nature, less ads will be displayed. If a keyword phrase is more commercial in nature, more ads (up to 11) will display on the search results page. How does Google change the number of ads? Well, if it is less commercial, it will tend to use less of the broad match criteria in displaying ads (which tends to bring up more ad inventory).
As you can imagine, this will affect impressions for ads, which affects your CTR and your quality score - ultimately your ads rank in Google.com. Also, advertisers will get more "qualified" leads this way. I also guess this means that Yahoo! will continue to lead in displaying more ads as a percentage of Web searches. Makes you think, doesn't it? Google is lessing it's ads, making the ads more relevant but will Google make as much money? Yahoo? well, they just seem to want to show more ads, and give it up to the highest bidder.
Forum discussion at WebmasterWorld.