An example of behavioral targeting in search ads. I first searched for [dubai hotel] or something, then searched for [famous hotels]. On the results of the latter results for famous hotels, the ads were filled with Dubai-related ads (during refresh or in another browser without being logged in to Google, these Dubai-offers weren't available):

http://blogoscoped.com/files/google-famous-hotels.png

Google announced this type of ad targeting earlier on:
http://blogoscoped.com/archive/2007-08-01-n50.html

PS: I'm not currently intending to go to vacation in a famous Dubai hotel :)