From Wiki article, as of April 20, 2009:

Summary
As of 20 April:

Boycotting the conference:
Australia, Canada, Germany, Israel, Italy, Netherlands, New Zealand, Poland, Sweden, United States.
Considering a boycott:
European Union, Denmark.
Members of the Human Rights Council that voted against/abstained of the conference, that has yet to make a statement:
Finland, Japan, South Korea, Ukraine.
Members of the conference’s Bureau of the Preparatory Committee:
Argentina, Armenia, Belgium, Brazil, Chile, Croatia, Cuba, Estonia, Greece, India, Indonesia, Iran, Libya, Norway, Pakistan, Russia, Senegal, South Africa, Turkey.
Confirmed attendance (non-preparatory committee members):
Czech Republic [now walked out for good — j.s.], East Timor, France, Ireland, Lesotho, Montenegro, Nigeria, Switzerland, Togo, United Kingdom, Vatican, Zambia.[45][46][47][48][49].[38]
Members of the Human Rights Council that voted in favor of the conference, that has yet to make a statement:
Algeria, Azerbaijan, Bahrain, Bangladesh, People’s Republic of China, Djibouti, Ecuador, Gabon, Ghana, Guatemala, Jordan, Mali, Mauritius, Mexico, Morocco, Nigeria, Peru, Philippines, Russia, Saudi Arabia, Sri Lanka, Tunisia, Uruguay.
Countries that walked out during the conference:
Austria, Belgium, Britain, Bulgaria, Cyprus, Czech Republic (has left the conference for good), Denmark, Estonia, Finland, France, Greece, Hungary, Ireland, Latvia, Lithuania, Luxembourg, Malta, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden and St. Kitts and Nevis.