VMware Fusion Tech Preview runs fine in ML, loads virtual machines from Fusion 4 and gives you the option to upgrade virtual machines to newer Tech Preview VM or run as they are. Have even been running Windows 8 in a VM in Tech Preview in ML. You can also load ML as a virtual machine in Tech Preview if you prefer.
link to beta of Fusion Tech Preview - http://communities.vmware.com/community/vmtn/beta/fusiontp2012
link to beta of Fusion Tech Preview - http://communities.vmware.com/community/vmtn/beta/fusiontp2012