A Search Engine Watch Forums thread asks how true is it that Gmail shows ads based on the content of the email.
Well, it is very true. That is their goal and that was the big stink when they launched. Google was reading your emails and placing ads based on the email text. What a privacy violation! Well, that big controversy died down, didn't it.
In any event, the ads Google shows on the right side and top tend to be relevant to the context of your email.
Some examples for you:
An email with the term "PageRank" in it shows an ad at the top for Google Sitemaps.
An email with the term "reciprocal link exchange" shows an ad for "Automated Link Exchange."
In fact, some emails that contain "link exchange," "text links," and so on have adds leading to Google's Webmaster Guidelines.
Nothing new, just not discussed much.
Forum discussion at Search Engine Watch Forums.