I've been running AdSense on my blog for almost a month now and I've learned some interesting things.
1 - though i've got four ads running at the same time on my blog, they are almost always targeted off the same keyword. for some reason, AdSense doesn't mix it up in the same ad block.
2 - my blog seems to generate three keywords; blog, venture capital, and spam filter
3 - venture capital is by far the most valuable keyword. blog is less valuable and spam filter generates almost no value. as an example, yesterday my blog was generating a lot of spam filter ads. I got six click throughs and a very high CTR of 3%, but made almost no money. That's because these spam filter ads pay so little. I make the most money when AdSense decides to run venture capital ads on my blog. The CTRs are good and so is the rate.
So I think there is an opportunity in these contextual networks to give the publisher (me) a bit more control over the ads that run on my site. I'd like to be able to mix the keywords in the ad block and even more importantly, i'd like to state a preference for which keywords run on my blog to maximize the dollars coming in and the relevancy to my audience.
Perhaps Google should build some tools to allow the publishers to do this stuff.