For the most part online advertising has taken the form of contextual and placement-targets ads with some ad networks experimenting with what is termed as interest-based advertising. Google through its advertising program has stayed with the first two options but according to a post on the official Google AdSense blog this will be changing over the next few month to a year.
Over the next few months we’ll start offering interest-based advertising to a limited number of advertisers as part of a beta, and expand the offering later in 2009. Whether the advertiser’s goal is to drive brand awareness or increase responses to their ads, these capabilities can help expand the success of their campaigns and should increase your earnings as advertiser participation increases.
Source: Google AdSense Blog
According to Barry Schwartz at Search Engine Land and the conversations he has had with Google’s Brad Bender there are currently 30 top line categories and an additional 600 detailed categories. As well users will have some control over which categories that what to see ads from by setting their preferences at google.com/ads/preferences.
As for when this part of their ad program will start it appears that over the next few months a select number of advertisers will be invited into the program. It will then be expanded over the course of the year.