Success! 
...but not the way we thought.
While waiting for ideas here in the forum, decided to open a support case. On the positive side it turns out that, at least, VMwF 5 Pro does come with a native OVF converter, eliminating the need for yet one more VM to run VMw Converter from. The less-than-positive side is that the converter is command-line based, so you WILL have to get your hands all dirtied up with bits, bytes and command lines... 
To quote from the support message:
Steps :
1) Open Terminal ---> Go to Applications-> Utilities ->Terminal or from the spotlight search for Terminal
2) Enter the below command
/Applications/VMware\ Fusion.app/Contents/Library/VMware\ OVF\ Tool/ovftool /(Virtual Machine .vmx) File / Path where it has to be saved
Example :
blr-gss-4th-dhcp-327:/ user(home)$ /Applications/VMware\ Fusion.app/Contents/Library/VMware\ OVF\ Tool/ovftool /Users/User(home) folder/Documents/Virtual\ Machines.localized/Windows\ XP\ .vmwarevm/Windows\ XP\ .vmx /Users/User(Home) Folder/Desktop/untitled\ folder
The above step would help to convert the Fusion virtual machine to OVF and the next step would be to Deploy OVF template on vSphere
After awhile, you'll find a new folder in the destination, named after the original VM and containing a .ovf file describing the VM and as many .vmdk files as drives the VM had. The vSphere Client readily takes the .ovf and lets you redeploy the VM on your big iron virtualizer. A few caveats:
- The folder will be as big as the original VM if not a tad bigger. Heed your space if constrained.
- Watch your versions. Turns out that the VM I needed to move over was in Hardware Version 9 as per Fusion 5, but my vSphere 5.0.0 barfed. So I had to downgrade the VM to Hardware Version 8 and it finally deployed.
- If possible, take out any peripheral you don't absolutely need to boot up (Optical drive, Bluetooth support, etc.) before converting and add later to the barebones imported VM.
- It will take a lot longer while to import/deploy than to convert! My modest 4GB Linux .ovf convert took the better part of the hour to find its new home on the vSphere. Even though the Mac was on GBit Ethernet on copper and the vSphere processors had a VNX5300 store on 8GB fibre!
Finally, for completeness sake, the OVF Tool folder inside Fusion (use Show Package Contents option if you want to go look for it graphically) offers a README with the following URL for more info on the thing: http://www.vmware.com/go/ovf/