I just solved the issue on both Windows 2008 guest and CentOS 6 guest.
On Windows, it is not sufficient to stop the firewall, stop also the "Base Filtering Engine" service. On Centos, stop SELinux (setenforce 0). For me, both are now working like a charm with a NAT interface to connect the Internet and a dedicated host-only interface to make the guest reachable from the host that MAC mountain Lion.
Hope this helps you