Included in this Patch Tuesday is a Forefront update KB2508823[1] (Client Version: 1.5.1996.0).

We have received a number of reports that the KB2508823 update fails during the install. Once the update fails, the existing Forefront client is also removed. This leaves the machine without any anti-malware protection.

We recommend you hold off deploying the update until confirmation from Microsoft.

Microsoft have posted a similar warning here:
http://blogs.technet.com/b/clientsecurity/archive/2011/03/08/fcs-v1-march-2011-update.aspx

[1] http://support.microsoft.com/kb/2508823

Chris Mohan