Do you click on advertisements in search engine result pages? If no, you might begin seeing less ads than you normally would. Barry Schwartz talks about a post on WebmasterWorld forum that explains how Google begun removing "top of the page" ads for users who don't click them.
In his example, he searches for "ipod" and refreshes the page ten times. After doing this, advertisements that were appearing at the top were moved to the side to make room for more organic listings.
Showing more ads to people who click them is actually a pretty good move on a couple levels. This must be Google's way of increasing click through ratios and creating a better user experience at the same time. For example, if ads are more likely to appear on the screen of people who click ads, CTR ratios could be a lot higher -- putting an even bigger smile on the faces of advertisers.
Not only are advertisers happier, but the users who make a conscious decision not to click ads will like the change too. This "feature" fits well into Google's plan to give users what they want (and not what they don't).