Today is the anniversary of the horrible attacks of the U.S., forever known as September 11th, or 9/11. People will be asking, why doesn't Google have a special logo up (a Doodle) to remember the day. The answer is because Google only has logos up for days that are lighthearted or fun. Today, is a special and very sensitive day for many people.
Other search engines are able to post something that does show some type of remembrance. For example, Microsoft's Bing and IAC's Ask.com have custom backgrounds for their home pages.
It doesn't mean Google is doing nothing. Google has a blog post named Make History with the National September 11th Memorial & Museum. Google partnered up to create a web site, using Google Maps API, named 911history.org.
Also, let's not forget the way the Google home page looked on the day of 9/11:
For more internet 9/11 resources, check out ResourceShelf.
So why no Google logo? It just isn't the right thing for Google to do today.
Forum discussion at Google Webmaster Help.