“Over the past several months, Microsoft has received reports on 4 different variants of the Conficker worm, the latest being Worm:Win32/Conficker.D (also known as Downadup.C, and the subject of a number of recent press articles labeling this variant as Conficker.C; see Win32/Conficker for a chart distinguishing the variants). 
In response to the previous variants of Conficker (A/B/C), the industry has collaborated to organize a community-based effort to help mitigate and provide relief to the threat posed by Conficker, known as the Conficker Working Group.
One of the initiatives employed by the Working Group (WG) was to prevent machines infected by Conficker from downloading additional malware. This was done by blocking access to around 500 domain names per day (250 for Conficker.A and 250 for Conficker.B/C) that these Conficker variants were programmed to monitor to download executable binaries. This effort helps to control the potential impact of the infected machines to cause further damage as the malware author pleases…”
For more information, check out:
http://blogs.technet.com/mmpc/archive/2009/03/27/information-about-worm-win32-conficker-d.aspx
HTH,
Tom
Thomas W Shinder, M.D., MCSE
Sr. Consultant / Technical Writer
Prowess Consulting www.prowessconsulting.com
PROWESS CONSULTING | Microsoft Forefront Security Specialist
Email: [email protected]
MVP – Forefront Edge Security (ISA/TMG/IAG)