If you visit Google, Yahoo or Ask.com today, you will see they are all honoring World AIDS Day with a red bow on their logo or on their home page. Bing, AOL, DogPile and others seem to be missing the mention today (we may put something up later).
Google has a ribbon under the search box which links to google.org/world-aids-day-2009.html (yes, Google.org):
Yahoo has an animated logo which links to yahoo.com/worldaidsday/:
Ask.com is the only one linking to a search result (go figure):
Forum discussion at Google Web Search Help.