Adsense in March of 2009. The new changes effect how Google collects information on your surfing habits using third party advertising vendors who leave cookies on your browsers - notably the Doubleclick DART Cookie.
The basic changes to Adsense revolve around the addition of Interest Based Advertising as opposed to the previous Placement Targeted Advertising. You can visit Google for a description of what Interest Based Advertising entails here.
As an Adsense publisher you also have the option not to use Internet Based Advertising and may Opt out by following the instructions posted in Google Adsense Help. I do not recommend that you opt out but the choice is yours. These changes should help increase your CTR and CPC in the long run as you will become less susceptable to being smart priced. (ie. Conversions should increase as users are presented more advertisements based on their interests)