A Microsoft spokesperson just confirmed to me that, although the company is still investigating "potential and existing vulnerabilities," there would be no regular monthly update this month.
For more information see http://blogs.technet.com/msrc/archive/2007/03/08/march-2007-advance-notification.aspx
that page states in part:
“We will though be making our regular monthly update to the Microsoft Windows Malicious Software Removal Tool.
We also will be releasing two high-priority non-security updates through Windows Update and Software Update Services and four high priority non-security updates through Microsoft Update and Windows Server Update Services.Because there are no new bulletins this month, we will not be holding our monthly TechNet Security Bulletin webcast originally schedule for Wednesday, March 14, 2007.”
The last time Microsoft failed to post security updates on the regular update cycle date was in September, 2005.