Reportedly on
http://www.chinadaily.com.cn/

etc.

http://www.baidu.com/

http://www.google.com.hk/logos/white_logo-276x110.gif

Also see: http://www.baidu.com/img/baidu_logo_jr_1008_zhouqu.gif

And Bing

http://www.bing.com/fd/hpk2/BlackCandle_ZH-CN190672519.jpg

And Yahoo

http://cn.yimg.com/hp/100420/yahoologo0420.png

Sogou

http://www.sogou.com/images/i_black.gif

You are so proud to share your discoveries but no one is glad enough to explain WHY all is black & white!

=> It's because it's a national mourning day in china due to the deads in the floods

http://www.google.com.hk/intl/zh-CN/logos/white_logo-276x110.gif logo with chinese sign

There was no good references yet when the first forum posts were posted. Personally I gathered information via local news (using AFP as source).

This happened in May 2008 too:

'Chinese websites turn sombre as nation mourns earthquake victims'
http://www.forbes.com/feeds/afx/2008/05/19/afx5023152.html

My source in this month:

http://yle.fi/uutiset/ulkomaat/2010/08/kiinan_verkkosivut_muuttuivat_surupaivana_mustavalkoisiksi_1904117.html

Examples are from search engines etc, there is China Daily's page:

http://yle.fi/ecepic/archive/00339/kiina_surup_iv__net_339659b.jpg