In an email I got about 3 minutes ago Google has claimed the following changes to the AdSense program.

Better and Better Ad Relevance
Now, in addition to understanding the overall meaning of a page, our technology determines the relevance of concepts found on a page to pick better performing ads.
Smarter and Smarter Pricing
With the new AdWords pricing model, we’ll also automatically adjust the price of clicks across the Google Network based on their expected value to the advertiser. A number of factors will be taken into account when calculating the expected value, including the keywords or concepts that triggered the ad. For example, a click on an ad for digital cameras on a web page about photography tips may be worth less than a click on the same ad appearing next to a review of digital cameras.

I believe that Google may use the clickthrough tracking offered from some AdWords customers to determine the value of a site as compared to other ad distribution (other sites, email distribution, and search results). Most likely this will cause a decline in click price as many large marketers have stated that contextual ads (on average) are worth approximately 30% the value of the same ad placed in search results.