VMware has a tool called ovftool. You need to do a full, custom install of Fusion to get the ovf tool. It is located in:

/Library/Application Support/VMware Fusion/ovftool

Change to this directory and run the following to see examples:

./ovftool --help examples```
To run a basic conversion use:

./ovftool path_to_ovf_file.ovf /output_directory/

This will convert the ovf into a format that Fusion can try to boot. This will not enable a virtual machine with incompatable hardware to boot.

As of VMware Fusion 4, the OVF tool is a seperate download and once installed it is located in:

/Applications/VMware OVF Tool