Internet Explorer is subject to a new exploit that has not yet been patched. It is worth noting that Vista running IE in Protected Mode and Server 2003 in Enhanced Security Mode (both of which are default behavior) limit the impact of this vulnerablilty.
If you manage a firewall capable of blocking access to known expoited sites, here's a list to block
If you manage a firewall capable of blocking access to known expoited sites, here's a list to block