Microsoft Office documents suddenly stopped being "safe to open"

Microsoft Office suite contains five memory corruption vulnerabilities in Excel program and another buffer overflow in processing "routing slips". A malicious Excel file or an Office file can exploit these vulnerabilities to execute arbitrary code on a client system using vulnerable Office versions. The specially crafted Excel/Office documents can be posted on a web server, file server, P2P share or attached to an email.

See: http://www.microsoft.com/technet/security/Bulletin/MS06-012.mspx