Issuing an out-of-band security update is a pretty rare occurrence. Must be important! Better go check now at: http://www.microsoft.com/technet/security/bulletin/MS08-078.mspx
No, seriously, go check now. It is important. Microsoft Security Bulletin MS08-078 is rated Critical for Internet Explorer 5.01, Internet Explorer 6, Internet Explorer 6 Service Pack 1, and Internet Explorer 7.