It's positive, of course You can read more at http://adsense.blogspot.com/2009/03/driving-monetization-with-ads-that.html
I just got the email. It sounds like they are trying to make adsense provide more relevant ads. Since it is beta I'm not sure when we will see its effect.
I just got the email too. Google says to update our Privacy Policy by April 8, 2009. I wonder in what manner (or wording) they require us to update our privacy policies?
I think it's better for us. It means that if your in a badly paying niche, ads could be served to customers with higher paying keywords. Thats good news for me! But in general, it should mean that users get presented with better quality and more relevant ads - this should see CTR improving, and subsequently eCPM. Also if advertisers are getting a better ROI more will advertise, and as such adsense should see more money put in!
This means that whereas before, your ad serving content was comprised mostly (if not completely, i don't know google's algorithems) based on the content of your site. So you could write about online degrees and get high paying keywords. Now, with this, the content is generated based on other site's content, that your end user has been browsing, and so there is one clear upside and one clear downside: Positive: If the user is into what he was just browsing, he will be more likely to click your ads Negative: if the user was just browsing a low paying keyword, he will click on ads of that nature, and so will other users. So you have to decide for your own site, if the potential low $$$ traffic is worth the potential more clicks. The poster above me makes a good point saying that if you have poor keywords in your site, this may benefit you more than those who do a good job of controlling their content for maximum $$ I am turning this off.