I don’t have official confirmation but this is what I observed when I was running a forum on the topic of investment (now sold).

You see the problem which I was facing was that whenever someone would make a new post, the ads which used to show were like – vBulletin Hosting, vBulletin Skins etc. In other words, totally un-related to my content.

I sort of expected this as every new post meant that the user saw a new url for the page. And oh, since the page was so big, so I bet Google used to pickup all keywords that it could – as fast as possible!

So my solution was simple — I had to get rid of all un-related words that could distract Google Bot on my page.

Ok I did it. I removed the word vBulletin from all places but the copyright notice. Then I changed all keywords that to could trigger a non-related ad.

Ouch! I was still getting vBulletin ads.

Then I realized that I hadn’t changed the default meta-tags. Voila! Keywords in meta-tags were triggering AdSense ads!

Realizing this, I changed my meta-tags to those related to my site topic and all of a sudden I was seeing all on-topic ads.

Which made me think that meta-tags do indeed help you target ads better and further experiments by me have shown the same.

On a sidenote, afterwards I discovered another awesome AdSense feature — Section Targeting.