That's right, and now I have proof.
I use Google Reader to read through all the different blogs and websites that I track day to day. One of the neat little features it has is the "Discover" link, where it looks at the feeds you're already subscribed to, and suggests others you might be interested in based on what other readers of that feed read.
Earlier this week I was doing some research to figure out how to raise the calcium level in my aquarium for my new Golden Inca snail, googling for terms like "raise calcium level in aquarium". Now would you believe it, that although I've never subscribed to an aquarium blog in my life, the next day when I was browsing through my discover page, there was two really good blogs on aquariums.
People keep warning about how dangerous it is for Google to have all this information on us, but I disagree. If Google knowing that I'm searching for information on aquariums means they can make more money (via targeted ads) and can implement jaw dropping features like this, all the more power to them.