Google knows who you are. It knows what you search for. It knows what you had for dinner last night and exactly where you like your back to be scratched (there, just between the shoulder blades, up and to the left -- ahhhhhhhhhhh). And, starting Wednesday, it will deliver ads tailored directly to you (yes, you in the Herman Miller chair -- stop slouching and pay attention).

In a blog post titled "Making ads more interesting," VP of Product Management Susan Wojcicki (aka, sister-in-law to Sergey Brin) describes Google's decision to move into behavioral advertising. To wit:

We think we can make online advertising even more relevant and useful by using additional information about the websites people visit. Today we are launching "interest-based" advertising as a beta test on our partner sites and on YouTube. These ads will associate categories of interest -- say sports, gardening, cars, pets -- with your browser, based on the types of sites you visit and the pages you view. We may then use those interest categories to show you more relevant text and display ads.

In other words, the ads Google displays won't just pull from the search terms you're using. Google will also look at all the sites you've visited lately. So if you're searching for, say, "baby wipes" and all you see are ads for porn, Google knows you've been a naughty little monkey.

[Note: Porn is not one of Google's officially sanctioned "categories of interest," but you get the idea.]

The concept isn't new; behavioral ad companies were all the rage a few years ago, which is why AOL, Microsoft, and Yahoo all bought one of their very own. But Google is the proverbial 8,000-pound gorilla -- when it does something, there's usually a boatload of banana peels to slip on.

There are limits, of course. Google associates the ads to a cookie in your browser, not your identity; so it will know about the naughtiness, but won't know which monkey is responsible. If you don't like the idea of Google delivering ads based on your surfing habits -- or you want it to know some of your interests, but not all of them -- you can change the settings in Google's Ads Preferences Manager. You can also opt out entirely, and install a plug-in for IE or Firefox that maintains your opt-out choice even when you nuke all your other cookies.